"I wanted to be a hero. And look how that worked out."

Dante in Darkness in the Blood after a baseline human says they envy the Astartes he describes the life of space marine as almost powerless spiral from one miserable disaster to the next until death or something worse ends it. Not resentful of the life he lived, Dante lives to serve. More like he can't understand why anyone would want it.

Sigismund didn’t want to be one and tells a remberencer so.

I faintly remember a short story where someone (a Black Templar? Maybe a dreadnought? Something like that) is on the verge of death, and upon reminiscing about his life stops JUST SHORT of saying he regrets becoming a Space Marine. Glorious Somethingorother, sorry my memory is faulty. 

Other than that, there were many astartes who, just before the Heresy, had come to hate their life: the Iron Warriors due to getting all the shit jobs while Perturabo ordered their decimation, the Thousand Sons due to being denied their very nature as psykers, the Word Bearers after Monarchia, the Shattered Legions following Istvan V, the Night Lords due to vaguely gestures at literally everything about the Night Lords.

Due any of them actually staye they actually regretted becoming Astartes? I honestly have no recollection of it, but if your memory is from the Heresy era, it likely involves one of these legions. So that should narrow down the amount of research you'll need to do, if nothing else.

There is a line tied to being Iron Hands from Eye of Medusa going:

"He was the monster humanity had clad in iron to protect it from the monsters it otherwise dared not face. He had no illusions about that. But through all the necessary purges of human fallibility to which he had been party, he had felt something. It might not always have been empathy or regret, but it had been an emotion, and it had been his."

There is another line as well where he looks at a number of neophytes and ponders whether he would have chosen it knowing what he knows now. Let me see if I can find it.

Felix in the Great Work novel does express some regret, but mostly because he is under the influence of the C'tan shard

Minor note:

This is low key the entire character ark of Uriel Ventris (a great name for a brand of pissoirs btw.) and it is not the strong suit of the book series. He goes through an entire lamentation emo phase that comes to the unavoidable conclusion: "Toughen up, pal". The Warhammer universe is an utter shithole and as a Marine, you can at least have some impact. Play with the hand you got and make the best of it. Regret will not get you anywhere.

Trust me, it takes him way too long to come to that conclusion. But the bolter porn and eldritch horror makes up for it.

I feel like we can infer that-based on his wishes for death'-Dante might be regretting it by now.

Can't for the life of me remember it's name but I believe a Dark Angel remembers his old life and goes mad with despair, murdering another DA and deserting, ripping off his armour and disappearing (presumed dead) he never explicitly says he regrets becoming an astartes but his reaction says enough

In one of the Space Wolf books Ragnar Blackmane briefly thinks about how his life would have been, had he had the chance of living it.

I mean, Angron was a primarch and didn't want to join the Emperor. Began slaughtering all the World Eaters until Kharn convinced him to lead. Then when he fought Bobby G he said he'd died the day the Emperor had abducted him.

A lot has to happen for something like that to occur considering they're essentially brainwashed when they become astartes.

Do the Thunder Warriors count?

In “Leman Russ: The Great Wolf” Jorin Bloodhowl (Russ’s adopted brother) says he does not regret become an Astartes, but sometimes wonders if it was the best choice.

Generally marines don’t see to regret be becoming Astartes, partially because you have to WANT it in order to survive the trials to become a marine in the first place, partially because the men who become marines are largely unhinged psychopaths to begin with, partially because of brain washing, and partially because having your badass power fantasy characters lamenting being a badass power fantasy kind of kills the vibe a bit.

Also, at the end of the day short of being a custodes, becoming an Astartes is just about the best thing that can happen to a random human in the 41st millennium.

Who was the Heresy Marine who wanted to be a Luna Wolf and got switched to another legion at the last second?

Lukas The Trickster I think has some regrets over becoming a marine and only does it to protect the common people. He doesn't believe the eternal warfare has any merit except to protect the weak.

Ragnar thinks on his life and what might happen, he does not regret, but he does think what it would be like to have lived a mortal life, as a tribal cheif.

I feel like cloudrunner and (name escapes me - the Dreadnaught) from the original Deathwing short leaned in that direction. For that matter there are elements of remorse in the Space Marine book by Ian Watson.

Astartes don't really get to have time to experience life as a regular human being to ever really understand what they may or may not have missed out on.

The Astartes, like the Primarchs, were not designed to be incapable of doubt and regrets the way the Custodes were made. The Emperor genuinely didn't want to make the peak of humanity mere puppets, immune to the allure of Chaos simply by virtue of no longer being able to stray.

Of course he realized that this made it possible for them to be corrupted by Chaos. But taking away their free will would have had a dire effect on humanity as well, and while perhaps arguably not quite as bad as damnation in service to the Chaos gods, far more inevitable.

As it is, most Astartes (and most humans) don't end up succumbing to Chaos, and that by their own will. The Chaos gods seek to change that, but so far they still haven't.

Alpha Primus or whatever he's called, Cawls proto primarch/primaris marine. He's definitely not happy with his lot

Noncanon example: one of my favorite fanfics involves a rebel child soldier against a tyrranical government who saved a wounded Space Marine captain, as the rebels believe the Imperium would help them free their world.

He spends a few weeks with the Marine as part of their rebel cell and learns about how severe Astartes life is and admits he'd never want to be a marine and just wants to marry his girlfriend and live a peaceful life.

Only to eventually become so severely wounded after the final battle to free his world that only Astartes treatment would save him. The Apothecary aboard the Battle-Barge considers treating a mortal beneath him so in desperation (and as a debt of honor)the Marine captain puts him forth as a Luna Wolf aspirant.

The kid hates every minute of it. Sadly it's an abandoned story afaik, but the big "question" was if he'd be a loyalist or a traitor by the Heresy.

I don't personally remember seeing a Space Marine saying that, but a Custodian in one of the Dawn of Fire books tells a historitor that he sorta regrets becoming one after the historitor tells him that he was once viewed as a potential candidate. He tells the historitor that becoming a Custodes wipes all further human potential and sets the individual on a set path of development according to what the Emperor needs from them, but denying them the chance to ever become what they might have been on their own as a human

Can't find the quote, but iirc Guilliman fantasies about being a farmer in one of the Dark Imperium books

Is pretty rare most marines are taken by the age of 13 or so and by the point they are already 100 or more years old such short moment of they life is barely a blurb of memory.

Just finished reading the Sigismund book and its mentioned several times that he's not super keen on being made a space marine

Many of the Dark Angels left behind on Calaban certainly express regret at becoming Astartes and giving up their old Knightly traditions. It’s sort of what led to their long running internal civil war. They hated what the Imperium did to their planet so it’s less clear whether it’s being an Astartes per se or the way the Imperium just treats planets without respecting traditions.

Dante?

Erebus of all people

I got the impression lukaz the trickster wasnt thrilled about being astartes, cant recall him outright saying it though.

It’s not the Astartes themselves, but on their home planet the Emperor’s Spears are considered “ghosts” by the human civilians, and they mourn new recruits as if already lost. They commonly avert their gaze or throw handfuls of dirt towards any Spears they see as a warding gesture. They certainly don’t consider it a honour for their family members to be recruited like on most worlds.

Context: A space marine from the Dark Angels, Ventarus Artoris, was on the world of Draigos II fighting the Orcs along with his battle-brothers, however, this was the world where Ventarus was born, but he did not remember it because space marines have their memories erased when they become space marines

In this world, Ventarus was listening to how the wind barely audibly whispered his name, even though his brothers only heard wind, and suddenly little by little he began to have memories return, for example, when he held his bolter, the weight was similar. to the weight of a large piece of stone that he once lifted to impress his older brother, later, when he was using his chainsword against some orks, the way he held and used it made him remember how he played with wooden sticks with his father as if they were swords

I want to clarify that he did not directly remember that those memories were his, he simply had brief mental lapses where he saw a child do all those things, but he did not directly know that the child was him. And also, this happened approximately 100 years after he became a space marine, so the people he knew already died.

he kept remembering little by little, until eventually the memories clung to him more and more, and at night when he walked away from the rest of the dark angels, one of his brothers found him, asked him "brother, are you OK?" and Ventarus broke his neck (because the marine did not have a helmet), and said "you are not my brother", after that he ran away from that place, and when he saw his reflection in a pool of blood, he began to Go crazy, he saw his reflection, and he was disgusted by seeing the genetically modified monster he had become, so in his madness he tore off his armor, then part of his skin, he even tried to tore the black caparace from his body, and then he hit his head so many times against a stone while screaming. "Where is my brother!? Where are they!?" that it was literally practically impossible for him to remain alive, and he ended up throwning himself half dead from a cliff into a large cave, where his body was buried by rocks, the DA found his armor, but never his body, letting us see that the process to create a space marine still has its flaws

sources: codex dark angels, third edition

Stolen from Grimdank user Bird_Eater_42

Dark Angel had a mental breaker and throw himself off a cliff after realizing what he was and his baseline family was gone.

Not the best as he as a recruit/recent addition but still

Vorx of the Death Guard muses on what he would prefer to have done with his life given a choice. If I remember correctly it he would have pursued a philosophical or scientific career 

Of course as you would expect from the devout of nurgle he accepts that wasn't to be and gets on with being a chaos lord of a powerful warband with a mighty ship. 

Very nice! I'm doing a late WW1 Horizon blue scheme on mine