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Literally that scene from Renegades: Harrowmaster
Reblogging because my response has become too long to put in the replies:
I don't think many people view the traitor primarchs as irredeemable, and I think the number among people who are well-versed in Horus Heresy lore is even smaller. They are, from the beginning, tragic characters. They certainly have their damning characteristics (namely hypocrisy), but they aren't really presented as irredeemable--we even have explicit statements regarding redemption for two of them (Big E says Mortarion can be redeemed in Godblight, and he outright forgives Horus in The End and the Death Vol III). I don't entirely agree with your interpretations, however. I don't think any of them are irredeemable (at least, not due to their own actions), but I think they miss some key details.
For all Angron claimed to be standing against oppression, he spent over a century expanding the Emperor's (extremely oppressive) regime. There's no moral purity shit here, either--Angron is pretty open about how he hates serving the Emperor even as he reduces worlds to ash in his name. What you said about Mortarion also applies to Angron, because of the Butcher's Nails. He was denied the chance to die with his family on Nuceria, and for the remainder of his life before daemonhood, the one thing he wants to do is die. And then Lorgar takes that away from him.
Mortarion's fall is because, first and foremost, he cared about his sons. Typhus brought the legion to Nurgle's garden, and infected them with the Destroyer Plague. Mortarion fell to Nurgle because he wanted to end their pain. Mortion is also a hypocrite. Like Angron, for all his claims of hating tyranny, he still expanded a tyrant's domain for a century and a half. (Corvus Corax is also on this list, but this is about traitor primarchs, not loyalist)
Magnus' guilt is very hotly debated among the community, and both sides have a point. On the one hand, he did the best thing he could with the information he had available. On the other hand, he was incredibly cocky, and did not have a concept of restraint. He believed he was in control, and he was too confident in that belief. (Side note: I despise the troupe of super powers being a stand-in for being queer (or any minority of any kind, for that matter)).
Fulgrim never said "fuck it, I'll have fun". His fall to Slaanesh was driven by his belief that, after killing Ferrus Manus, he was irredeemable. He's an addict; he isn't trying to have fun, he's trying to forget reality.
Everybody agrees that Big E mishandled Lorgar, and that's the most favorable interpretation for him. His argument on why he isn't a god is literal abuser language ("If I was a god, I'd use my psychic powers to force you all to kneel before me, like this" before using his psychic powers to force all of the Word Bearers to kneel before him). And all this ignores the shit Kor Phaeron did to him.
Horus is the spoiled rich white boy of the primarchs. He didn't fall because he was lost because dad was doing everything for him beforehand, he fell because he saw a vision of the future where he wasn't given the credit he thought he deserved. And then, like anybody else who grew up being told the world would be at their feet, he lost it. There's other issues as well, such as taxes (Terra wanted to impose heavy tithes on newly conquered worlds, which Horus opposed because it would cause them to rebel), but "gee, maybe I'm not ready" was never an issue.
Alpharius and Omegon believed that they knew the Emperor's plans better than he did, and threw in their lot with the traitors because they thought that if the traitors won, it would mean the end for Chaos.
Perturabo is someone who chose to suffer in silence and hope things would magically change. He never complains, and then is shocked when things don't go his way. I'm fairly sure he's also the primarch that people find the most relatable.
Konrad Curze was certainly mentally ill, and he was certainly denied the help and support that he needed, but I don't think he was abused because of it. He's also the kind of person who thought that if you publicly lynched enough jaywalkers, crime would go down, and that the Emperor sending an assassin to kill him because he committed treason and genocide was vindication for his belief that any crime warrants being skinned alive as a punishment.
They may not be irredeemable, but their hands are fare from clean.
I love how much of warhammer 40k is clearly a bunch of white dudes sitting around, thinking up the worst shit imaginable and going "thank god that could never happen to me, the cishet white guy"
angron is a one for one depiction of slavery, yet is painted as the bad guy for wanting to stand against oppression
mortarion is disabled and constantly has his decisions taken away from him by able bodied people only to be seen as moody and uncooperative
magnus is gay or trans or both. literally a guy blamed for something he was born with that he cannot control and told you're bad if you explore this part of yourself. also you're illegal now
fulgrim was actively encouraged to pursue perfection, despite never being good enough for others. he pushed any personal pleasure aside for an uncaring crusade and then is demonized for saying "fuck it, I'll have fun"
lorgar. also known as "this is why you don't abuse your kids"
horus is the golden child who's sent out into the world by himself only to find out Gee, Maybe I'm Not Ready because good ol dad did everything for him then told him "figure it out lol"
alpharius omegon are the autistic kids who don't understand why pops is doing this, maybe we should do things a little different than "blood soaked crusade"
perturabo is the burnout middle kid who did everything to impress his father only to be told "that's what's expected of you" who then got mad since acting out was the only way he got attention
konrad curze has a mental disorder. and is abused because of it
all of these characters were so close to being some of the best representation for minorities we could get in media only for gw to eat shit right at the finish line because they can't commit to an actual story. and it's amazing just how little these writers understand that the things they depict are all real things that people suffer through every day, and are demonized for every single day, especially when we are told these characters are irredeemable and should be destroyed, no questions asked