I think you're proving my point, though. You hate racists. That's fine, but let's say your best friend from childhood is a racist. Do you stop associating with him before or after he says “nigger” in your presence?
I stop associating with him after I tell him, at least once (and depending on how long and the deep the friendship is, maybe several times), that I find that word offensive and that if he wants to be friends with me, he needs to stop using it (and better yet, change his attitude).

If I were black myself, I think being called a nigger once would be enough to end the friendship, which is essentially what Snape did to Lily.

That's my point; Lily was already looking for a reason to dump him. Should he have said it (at any point, to any one, for any reason?) No. Do I think she was justified in cutting ties with him? Yes. Not BECAUSE he called her Mudblood, though, but because he was heading on a path she could not follow. And I still maintain that had she given him an ultimatum, he may have changed his ways.
I think there is absolutely no evidence to support the idea that Lily was "looking for a reason to dump him," and it looks rather more like she was looking for a reason to stay friends with him, and increasingly finding it hard to do so.

Someone who goes that far down a dark path almost never comes around because the girl he's been crushing on (but he knows is never going to be "his" in the way he wants even if they do stay friends) says, "Stop this or we aren't friends anymore." I also think it's unreasonable to expect that Lily had any obligation to do more than express her feelings on the subject. It was never her job to save him.

Part of what you're claiming he needs to be "redeemed" for is that he was never properly cared for by parents who obviously never wanted him and had no business being together in the first place. This kind of "well if the parents don't care for the child why should I" mentality makes me crazy and is brutally unfair to the child.
Wow, that's quite a mischaracterization of my position. I certainly don't think, "If the parents don't care for the child why should I?" I have a great deal of sympathy for the young Snape, and for any child who grows up in an abusive, unloving home, and I realize all too well how hard that makes it for someone to grow up to be an emotionally healthy person. However, having lousy parents and a miserable childhood is never an excuse to abuse other people in turn.

Snape loved and respected Lily (at least at one time), which means he was able to recognize something in her behavior that was good and admirable. That means he was able to perceive that there were other ways of treating people than the way he learned at home. Yet he chose to follow a path that he knew led towards abuse and violence.

What I'm claiming he needs to be redeemed for, by the time he is an adult, is (1) Joining Voldemort, whom anyone not completely stupid or sociopathic could see was a sociopathic madman bent on murder, torture, and oppression, (2) Selling out his best friend and getting her and her husband killed, (3) Turning into a horrible, abusive teacher who bullies and emotionally traumatizes eleven year-old children because they remind him of people he doesn't like. Say all you like about what an annoying know-it-all Hermione was, or how Snape might have been justified in hating James: the way he treated Harry and Hermione in class, to say nothing of how he treated Neville (against whom he had no conceivable justification for animosity) was enough to make him despicable all by itself. You can't justify all that by saying, "Well, he had a lousy childhood, so he didn't know any better."