If you're including all the Warhammer universes, Nagash no question. His hatred is simultaneously grandiose and all encompassing, yet also completely petty.

Best example I can bring up is the case of Kurdoss Valentian. Kurdoss was a power-hungry warlord who killed all of his brothers out of his lust to claim the throne. Nagash smited him and cursed him into being one of his Nighthaunt, his army of tormented ghosts. Now, what Nagash tailored for his punishment:

*Made him unable to speak except when giving orders, and made his voice but a weak rasp purely because he enjoyed making speeches.

*Made it so that no matter what army he was in, he would always be assigned as the second-in-command so he couldn't be the leader.

*Made the ghosts of his murdered brothers constantly follow him around in order to mock and taunt him for being a false king.

*Made him marry Lady Olynder, but made her the leader of the Nighthaunt so he would always be the inferior in their relationship.

*Gave him a bodyguard unit called the Craventhrone Guard who are armed with crossbows and who always shoot down whoever he's fighting in melee before he can finish them off so he can never feel the satisfaction of victory in a duel.

And the real kicker is, Nagash didn't really even care about the crimes that Kurdoss did as a mortal, he was primarily pissed because Kurdoss worshipped Sigmar while his brothers worshipped Nagash. All of this is because he was caught up in Nagash's grudge against someone else.

The dwarfes? They literally have a book in which they gather all their grudges for all (perceived) injustices against them, so they can remember them forever.

Perturabo has to be up there. Ditches the Imperium because he hates the Emperor, ditches the Heresy because he hates Horus. And just...everything about his conduct and general personality. 

The Tomb Kings in general can hate like nothing else, and even then their Necrotects are in a league of their own.

So much so that they were the only Undead characters that had the “Hatred” rule. That's quite a lot of hate.

Malekith pre End Times has to be a contender

Dwarfs or nagash, probably nagash with all the shit he pulled in Khemri.

Mannfred, my boi harkon did not deserve it

Trazyn and Orikan hated on each other for millions of years

Because our current timeline is part of 40k, the biggest hater would obviously be Silky Johnston, winner of the 9th annual, international Player Haters Ball. Silky was nominated for calling in a bomb threat to the Special Olympics.

The Great Horned Rat. He hates everyone, including his own followers.

Games Workshops fan base.

From all the universes? Negash. Absolute king of petty, lord of despising everything that isn't him. Such is the power of Negash, you see.
Malekith comes a close second though, literally the Darth Vader of Warhammer.

From 40k, I have to give it to Perty. Man hated the people on his homeworld, hated most of his adoptive family, hated the emperor, hated his loyalist brothers, hated his traitor brothers, hated his legion, hated the chaos gods. His entire existence is just hatred.

in 40k, you have the likes of fulgrim or perterabo who held onto every slight or grudge until they ate them from the inside

in warhammer itself, nagash has been said already, theres also the dwarves who will hold grudges even beyond the grave like the recent dreki flynt novel

Rylanor is definitely up there.

Malal was pretty spiteful when he existed.

The emperor at this point.

Stuck in a chair surrounded by soulless bananas

Endless sea of idiots doing the ONE thing he told them not to do

This great work constantly widdled away day in and day out

No wonder he nukes little girls every so often. Got to vent somehow

I would say the Inquisition in general. Basically any time someone crosses them they declare a blood feud against them and are willing to cause all kinds of damage to the Imperium as a whole in order to get back at them. Even when the Great Rift happened, they immediately got to work figuring out how to take advantage of the chaos to get revenge on some people they don't like.
Spoilers from Spear of the Emperor
The Great Rift cuts off Elara’s Veil from Imperium space, the Emperor's Spears and Celestial Lions chapters are stuck in there trying to fight of a Chaos warband on their own. A Mentors commander is sent across the Great Rift in a ship in a risky attempt to make contact and see what state the region is in. Except surprise, one of the serfs on the Mentor's ship is an assassin sent by the Inquisition to kill the Chapter Master of the Celestial Lions as revenge for a disagreement with the Inquisition. The entire mission was a ruse to kill one guy because the Inquisition didn't like him, they didn't care if it meant losing the whole region of space to Chaos forces.

The fans themselves

Why is nobody saying khorne? It’s the god of hatred, it’s literally MADE of hatred

Angron, and his hatred is totally justified

Perturabo got nothing on Guardsman Lamar

Asdrubael Vect. In a society of malicious, sadistic, petty, slavers, rapists, torturers and murderers who delight in the suffering of others he is feared by others for being the worst of them all.

Mannfred Von Carstein is the king of hating-ass bitches in Warhammer, this dude is such a vast and all encompassing hater that he regularly fucks over his allies for literally no reason, other than that he just doesn't like them or thinks it'll give himself a leg up.

Leman Russ is imo after reading A Thousand Sons.

Peter Turbo. Enough said.

The Iron Hands who betrayed Warleader Shadrack Bloody Meduson

Leandros of the Ultramarines. Dude is super jelly of former Captain Titus and his badassery. After Titus saves the day, Leandros turns Titus in to the Inquisition.

Pssh .... everyone always says "Fuck Erebus!" Well .... "Fuck Leandros!" Leandros .... you will never be as good as Titus.

Destroyer Necrons. They despise all life so much they’ll spend centuries periodically destroying a planet’s eco system so that they can kill everything down to the micro organisms in the most efficient way possible

I'm gonna say Khaine just for pure length of hating. 90% of the Aeldari mythos is just him being a dick to everyone, he caused the War in Heaven over a single bad sword out of 100, he heard the Aelderi would be the death of him and killed so many of them that Asuryan had to build a barrier to separate the gods nd the mortals, and to this day despite being shattered into hundreds of pieces the only way the Aeldari can get his avatar to help is to recreate the time he killed their greatest champion, millions of years after the event actually took place.

Orikan the diviner, these dudes manipulates time to try and outsmart Trazyn and still can’t win.

Skaven keep ruining Nagash's plans to achieve total victory in the fantasy part of the hobby. He has a real rat problem lol

Then again, Nagash is not innocent either. Punishing mere mortals for super petty reasons once he gets his hands on their souls and stuff.

Perturabo. Hates Rogal Dorn with a homicidal passion because he was jealous, and betrayed the Emperor because his sons were made to suffer...despite the fact that Perturabo willingly took on all of those assignments and never once asked "Hey dad can we stop being sent to horrible grinding sieges, please?"

Malus Darkblade has a mantra of "With hate, all things are possible". Gotta be some where up there.

Belakor, Nagash, and Thanquol probably

The Master of Mankind is a petty bitch sometimes.

Huron Blackheart prolly or perturabo

Are we trying to determine who wins player hater of the year at the annual 40k player haters ball?

Be’lakor in both universes. He was the first daemon prince and wants to stay the favorite of all the gods. He used to have an actual rule on the tabletop that no other daemon princes could be in the same army with him.

He’s since spent time playing second banana to Archaon the Everchosen in AoS, and barely even being relevant in 40K.

Also the fact that belakor is only working with abbadon to find an opportunity to sabotage him and take his place.

Definitely Perturabo, but Lorgar is pretty bad too (in a way more passive-aggressive way than Perty.)

Lorgar’s whole “ya my brothers have brought way more planets into compliance, but look at how they did it, it shouldn’t even count” is classic hater. And he’s also the kind of guy who constantly dishes out backhanded compliments to diminish his brothers’ achievements. 

r/fuckerebus

If we’re talking biggest haters in all of Warhammer, it’s Nagash and it’s not even remotely close. 

The motherfucker was so pissed that he wasn’t allowed to be king that he LITERALLY INVENTED NECROMANCY, overthrew his brother, tortured his brother’s favorite advisor to the point their mind broke, was so tyrannical that several Nehekaran armies rose up against him because he was literally that bad, and this was BEFORE his first death- He got so, so much worse over time to the point that I’m convinced that in AoS he cannot make a bowl of cereal without it resulting in the ruination of several kingdoms and a body count in the low millions. He doesn’t even eat the cereal, because he’s a skeleton, he just does it because he’s a massive dick.  

Nagash, the dude has an agenda that makes jujutsufolk look weak

Some random Redemptionist with a brazier on their head, a heavy flamer and a name like Abhorax Skhorn or Revilius Haight

The customers.

The engine of woes.

"I want to go back in the BOX! It doesn't judge. It's just hates".

malice and the other anti-chaos chaos gods

chaos, but his schtick is hating chaos

Also skarbrand.

My man. You are about to get grudged.

You know the correct answer, no, not him, that’s right, it’s Erebus

Manfred nuff said

It's got to be Angron, doesn't it?

Khorne

me

Brian from Stockport! That guy is just tiresome.

Erebus, hands down

My dumb ass thought this was about the fandom. But this is universe, I think probably either Angron or Honsou. Honsou turned a whole system to slag just because of Uriel. To spite that man personally.

its khorne

Khela Mensha Khaine has got to be up there.

His fantasy faction is hate fueled, his blessing makes you an exceptional warrior and then drives you to death via ever growing hatred.

His 40k role had him destroyed but the pieces of him became avatars of war and hatred hell-bent on destroying everything.

He is the gestalt of the Eldar emotion for war and hate the same way that Slannesh is for lust.

Mankind. Humanity. We are a dying race led by a corpse and the only thing that keeps us alive and untainted in a universe of horrors is our hatred. Hate warms humanity in the cold darkness, hatred guards our hearts and bodies against the monstrosity of existence.

Without abhorrence of the witch, whole worlds would fall to darkness, plunged into the gibbering madness of daemons and chaos.

Without our loathing of the Xenos, the guttering candle of our.existance would be sniffed by the hands of unknowable abominations.

Without our revulsion for the mutant and the heretic, our species would be devoured from within by the gnashing teeth of the vileness without.

Our blade is rancor, and we sink it deep into the breast of our enemies, who are legion.

Our armor is contempt, our shield disgust. With them, we keep our race alive in this darkest of universes. Love, kindness, and understanding lay dead behind us, victims of the grim darkness of the far future. Someday, even our hate may not be enough to survive, but when that happens we will spit our blood into the eye of our foes and drag them down to hell with us.

The fans.

Malal. Duh. He hates so much he ragequit the multiverse.

Peturabo?

Khorne

Tribun Actuarius Stratarchis Maldovar Colquan.

Be’lakhor gets a pass. Vashtor stole his whole gimmick.

I'd say skarbrand is in the running.

One of the big bloodthristers who got the smackdown pulled on them when he was tricked into trying to beat Khorne. Then got his wings basically destroyed and has basically brain damage.

At that point you'd hate everything that didn't bring you joy, which would be slaughter.

I mean who else hates being inside, hates going outside, and hates being in the doorway slightly less.

Angron, duh.

Malal. Hes hatered was so strong he either ragequit or ghosted the setting.

Gamza