Whenever I hear the argument "Snape only changed sides when it personally affected him" as proof that he was an irredeemable character, I cannot help but think, wasn't that kind of what Lily did too? Lily "But you call everyone of my birth Mudblood, Severus" Evans cut off Snape only when he personally offended her. She was fine with being friends with him, even when he supposedly hurled slurs at other people of her origin, as long as he didn't do the same to her. The way Snape has no retort to this, and the way Lily remarks it as an unassailable and conclusive argument, suggests that this is the first time she has addressed the issue and called out Snape for it seriously.
“Slipped out?” There was no pity in Lily’s voice. “It’s too late. I’ve made excuses for you for years. None of my friends can understand why I even talk to you. You and your precious little Death Eater friends—you see, you don’t even deny it! You don’t even deny that’s what you’re all aiming to be! You can’t wait to join You-Know-Who, can you?”
But why are you making excuses for him if he's being that hateful and bigoted? How can you be friends with someone who is insulting everyone, for the reason that's mutual between you and the victims, except you? Lily's final straw wasn't Snape's association with future Death Eaters or even his aspirations to join them, not even Snape targeting every other muggle-born except her, it was Snape causing her personal inconvenience, which, rationally should have happened the moment he started displaying Death Eater tendencies and in the case of Snape, should have happened the moment he realised how his friends harboured prejudiced beliefs against people like his friend. In both cases, they should have severed ties with their friends long ago. To be friends with people who mistreat a certain group that you, or people you love, belong to, is already a questionable action, and then cutting them off from your lives only when they go against you individually displays selfishness, but why is only one of these seen as the ultimate immoral deed ever in the entire series? If one of the two is inexcusable, then the other must also be deemed condemnable. Conversely, if one act is justified, the other must also bear justification. Is it politically correct? No. Is it morally ambiguous? No. Is it very human? Yes. That's the point, both of them stayed with their respective friend groups because they individually felt accepted and loved, and the toxicity of those people didn't sufficiently deter them. It took both of them too long to realise that they are not necessarily protected solely by their relationship with those problematic people. If Lily failed to consider that one day Snape might turn against her as a result of his indoctrinated beliefs just because he was her close friend who used to care for her and had influenced her life positively some way or the other, then the same can be said for Snape, who was neglected at home, bullied by a gang of hailed "righteous" heroes and felt abandoned, finally found a place to call home through these people and where he felt welcomed, might have underestimated the harm such individuals could inflict upon him, whether emotional or physical.
