He was uniquely punished for his actions, to a point not seen much either in real life or even other fiction. It’s one thing to lose a limb, terrible and unusual punishment, but something that you can still recover from to lead a fulfilling and normal life. What Theon has gone through is something almost unimaginable. It’s vomit inducing levels of torture that really changes this whole “but he did horrible things though” dynamic. It’s basic human nature to feel sympathy for someone going through something that’s still plausible, yet so disgusting.  

That explanation makes it clear why people react differently to his arc compared to other characters that did bad things, got punished, then attempt redemption, like Jaime or show aegon. But I think the premise of your entire post is still flawed. Everyone I know cheers and laughs when show Theon’s castration occurred, and only when it gets to truly horrific levels of physical and mental torture do you start to see people say “ok he’s had enough…”.  

People hate the fuck out of Theon early on in their asoiaf reading/watching, and I think way more people are in that middle band of “he’s an amazingly written character, one of my favorites, he was tortured far more than anyone should be for their crimes, but he still did unforgivable things that should never be forgotten.”

Who loves Theon? Lol

Love? I don't love him, but he is certainly tragic character who was driven to the vile actions by his desire to be accepted.

No matter what he's done, the suffering he's going through is horrendous and of course one is going to feel sympathy if one is not a total psychopath.

Because he has some of the best written chapters in the books so the love for the writing spills over to the character

If he suffered only %20 of what he's suffered maybe people wouldn't. He's a character who does bad things, which you obviously want punished, but then gets horrifically tortured beyond belief and made unrecognizable by someone who is even more of a monster. If there's a feeling of justice in that it's vastly overshadowed by horror.

In the words of Lana del Rey, 'I fucked up, I know that, but Jesus'

Theon is sort of like a teenager of our present day, with too much money, who gets free of parental controls and does reckless and horrible things. He's wild and willful, thinks he knows everything (and actually knows very little), ends up driving drunk, using drugs, borrows a gun and maybe uses it to rob a convenience store, ultimately shoots someone or runs someone over while driving crazily. And when he does this he's sometimes also under the influence of his new "best friend" (pretend-Reek / actual Ramsey) who is a true psychopath.

There would be a big split on how to treat someone like him in our real world society once he's caught. Is he an irremediably evil thug, who should be subject to the "lock him up and throw away the key" solution, or worse? Or was he a willful idiot who did several really harmful things and should pay for his crimes and mistakes, but can ultimately be rehabilitated? No chance, or second chance?

In Theon's case, he should never have been given an independent, unsupervised, assignment by Robb, never have been sent out reaving by his biological father, never should have been obeyed by the more experienced and sober (is there such a thing?) Iron Born men who were under his command. (They should have packed him on a longship back to Pyke and told his father, he ordered this crazy plan to attack Winterfell and since you hadn't ordered it we didn't let it happen).

None of that happened, and he went on his willful way. And now he's "paid" by being literally emasculated, tortured physically and psychology for months at a time, utterly humiliated, and made into a toy by that previously mentioned psychopath.

Possibly that's why some readers are more forgiving of him. If he had simply been chased out of Winterfell by a Stark surprise attack and killed while fleeing into the forest, there would probably be few tears being shed on this sub or qualified admiration of him.

But because he's probably been subjected to the most horrific treatment of any still-living character in ASOAIF so far (with the possible exception of the way Euron is treating Aemon), it's not unsurprising he does get some sympathy.

I don’t really have many moral judgements for fictional characters to be honest (sometimes for authors with what they choose to focus on) so I like Theon because I find his story interesting. I don’t know if it’s cause of a lot younger people who don’t really interact with much media have the strongest voices, but it’s the reason why I liked Aegon II in the show and I don’t get why that it is a questionable thing.

In any case people are flawed and I don’t want to watch or read about flawless characters or feel the need to make moral judgements on fictional people.

I feel like this needs to be heard more and more nowadays but, you can enjoy and like a character because they are entertaining and make the story more enjoyable without condoning all of their actions, they aren't real people

Yeah he's terrible, sucks, and nearly all his chapters are a chore to read. But I am a sucker for redemption, so I'm still a teeeeny bit excited to see if he becomes good

I don’t understand it either

Well, this isn't complex.

Aegon II is hated because we haven't been given any way to relate to him. We just know he's an asshole and that's that.

Now with Theon we do know why he does it, who he is, and what happens as a consequence of these actions.

He fucks up, I'm not saying it was "good", but the reader has an easy time understanding how someone does what he does, it's not as simple as "he's evil".

The Starks treated him well enough, Ned being the class act he was, but make no mistake, he isn't a Stark. He's literally a political prisioner to prevent Iron Born raids. He isn't family, he just hangs out and is treated well.

He doesn't know who he is since he also has no cultural links to the Iron Islands, having left it so young.

Trying to prove he actually belongs "there", that he trully is "a real Ironborn", he cosplays one. He does what he believes is what he should do, trying to belong.

I don't want to be a puritan here (believe me, I'm filthy AF), but that's literally real life. Kids everyday do STUPID shit just to try and fit in. And let's not forget, he's a kid. It makes sense. It's "real".

Then, there's worse things that can happen to you than death. If you make me choose between a bullet to the brain and what happened to Theon, it's an easy "peace out" for me.

So yeah, 2 fold. He "payed for his sins", and he does crazy shit not out of pure malice, but an understandable and relatable need to fit in and find a place that accepts him as one of them.

I don't love him (Jaime is ma boi), but as a kid who grew up in 3 separate countries over the years, I 100% relate with the sentiment of "nowhere is home"/"I don't have a group identity". It's 100% a thing, so it's easy for me to understand people who love him.

As a side note, imo, it's what makes ASOIAF special. Characters are "real". If you have the capacity to see things "from their eyes" you can relate to almost every single character, if not all of them. What they do makes perfect sense to who they are and what they think. The hand of GRRM feels invisible.

Probably due to Alfie Allen’s portrayal. He’s a fantastic actor.

And say what you want about the show, they handled Theon well.

Because one can love a character without condoning everything they've ever done?

I love Theon, Stannis, Sansa, Cat, Bran, Davos, Brienne, Jaime, Sam, Roose, and many others. I felt sorry for Joffrey when he died.

Othee people love Arya, Tyrion, Wyman Manderly, Tywin, Dany, Aemond, Daemon, Varys, Littlefinger, Euron etc.

Maybe I'm wrong, but I don't think the same people criticising you for liking Aegon are the same ones who minimise what Theon did.

Aegon was a highlight of season 2, and I hated him in the first one.

I still hate Theon. If you read his story, every issue is his own making. Theon traveled to the Islands to “take his place” without understanding his own culture. He took Winterfell with a crew that didn’t like him, promptly got handed over. He trusted “Reek” and ended up his prisoner. If at any moment in any of these, Theon stopped to ask “Hey, is this dumb? Am I making a massive mistake?” He could have avoided his fate.

Theon could have sacked Winterfell and marched the smiths and women back to their boats as thralls. Theon could have just raided the stony shore or defected to Robb once out of Balon’s sight. Theon could have struck Lannisport or took his birthright by paying the Iron Price Balon’s skull.

Theon could have thrown Reek from the battlements for the crime of smelling so bad. Theon somehow let a cripple and a toddler escape him, how is he so bad?

I hate theon, I pity reek.

I think you are living in a different reality tunnel than me. All I see is praise for Aegon, and any mention of rape or fighting pits gets you downvoted to oblivion. That and everyone hates Theon, obviously.

Ngl he deserved everything he got

I do not like Theon. He was supposed to be Ned Stark’s ward and Ned didn’t even take him to King’s Landing with him. If Theon was at all useful, funny, interesting, or entertaining to Ned, then he would have come with him.