"He called Lily, his best friend, a slur-" When my parents and my then best friend/now girl friend found me, after I was violated and was in the most humiliating position, I called and wished them worser things. Things I didn't actually mean. You'll don't understand, that when you are in such position the last thing you'll want is, people you love witnessing you in one of the most horrifying and embarrassing situation all powerless and violated. Lashing out is NOT uncommon. Because you were powerless in that situation, you take your anger, bitterness and pain out on people you love while trying to hold on to the scraps of dignity and self respect left in you. I don't why you'll act like you'll haven't hurt your loved ones with your words before. If you'll haven't, good for you. Some of us aren't that perfect and we get angry, lash out and say shit we don't mean. What matters is you ask them for forgiveness (if they deserve it) later after you calm down. Which Severus did. You'll shitting on characters for showing realistic, human reactions is wild to me.
The people we love are more able to hurt us than the people we hate, and while they may not be actual threats, when your brain is in that particular panicked, reactive space, there's no room for nuance and anything present in that space gets flagged as a threat and your immediate motive is to get them out of there, often by lashing out to chase them away.
Also even if they are loved ones and don't mean any harm, their witnessing of something you don't want them to see is a functional harm, and if they're forcing their proximity on you under the guise of care, that's an assault and a violation of agency and a mindfuck all crammed into one uncomfortable package.
Lily was: 1) witnessing his humiliation, after 2) inserting herself into the situation 3) badly 4) for primarily her own enjoyment of insulting James, all after 5) ignorantly helpsplaining her uninformed theories about The Prank and 6) dismissing his attempts to tell her some semblance of the truth as jealousy, obsession, and undue hostility talking, as part of a larger pattern of 7) downplaying her housemates' longstanding campaign of bullying and persecution of him.
Oh, and 8) objecting to his other friendships while 9) applying a different standard to, again, his longstanding bullies.
Like, it never ceases to amaze me how much stuff Rowling put in her stories without noticing, but Lily is absolutely not being a friend to Severus here, and while it's absolutely her right to cut ties and to have whatever problems she wants to have with him or anyone else, the fact is that she's dismissing all of the pain and distress he's in, championing his bullies, effectively gaslighting him about his own experiences, and subtly threatening to cut ties with him to push him to cut ties with his other connections.
And then she does an embarrassingly bad job of defending him, handing James two separate hooks to sink into Severus (an invited soapbox to announce what he finds objectionable about Severus, and an opportunity to sexually harass and extort Severus' friend in front of Severus, throwing Severus' inability to defend his friend in Severus' face.
The third hook, then, mocking Severus for needing to be defended by a girl, ends up being the straw that breaks the camel's back; his response is full of pent-up hostility towards someone who he really wishes wasn't doing anything she's currently doing, and who has demonstrably refused to listen to his priorities at all for the entire conversation.
It is a demonstrated phenomenon that inept allies often inspire more contempt and resentment than the actual enemy. If you go in to defend someone and you make such a hash of it that you embarrass them, they're going to be mad at you and they're going to be justified in it.
(The other half of the blood purity argument is that muggleborns are outsiders, who don't know how the culture works and therefore make asses of themselves. Draco's initial discourse to Harry on the matter had nothing in it about diluting pure wizarding bloodlines but was all about "they don't know anything about our world" and related concerns. As such, its use by Severus against Lily carries more connotations of ignorance, carelessness, and mucking up a battlefield she doesn't know how to navigate but insisted on directing anyway, than assertions about the nature of her ancestry.)
TL;DR Severus is a raccoon caught in a trap here, and Lily's come in and is trying to "helpfully" yank him free, causing more pain in the process, and OF COURSE he's going to bite her.

















