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I had a thought: What if Eileen had been just as bad as Tobias?
Snape hated muggles mainly because of his father, and I'm gessing that his love for his mother aided his idealisation of the wizarding world. Maybe she was good, or perhaps neglectful, point is, she wasn't as bad in comparison to her husband, which probably made her good in Severus' eyes.
But what if she was just as bad?
Would Severus hate pure wizards too, then?
Main reason he chose Slytherin was his mother too, so he probably wouldn't have cared in which house he ended, or perhaps he, like Sirius, would have hated that house.
I think he still would have been interested in dark magic, but I don't think the others in Slytherin would been able to get to him so easily, even if he ended in that house again.
I can actually imagine him remainig neutral to spite everyone or helping the Order because Lily was a part of it.
Also, I belive the marauders would have ignored him in favor of some other Slytherin.
I can picture the scene in the train, with James asking him which house he wanted to go to, and Severus merely shrugging in response, maybe suggesting Ravenclaw as a probable ending.
From there, James and Sirius probably wouldn't care about him, and when Lily ended in Gryffindor, well, the lions and the eagles tend to get along alright.
And this Severus doesn't like girls, simply because I say so. But even if he did, I don't find much logic in him falling for Lily. There is a moment in friendship when you simply know the other person a little too well to fall for them. And that is something that would have to be practically beaten into James skull, but they would get there... eventually.
And maybe Voldemort would actually try to recruit Severus, but I don't see that succeeding.
The lake prank and the werewolf incident wouldn't happen either.
So... I'm probably looking to closely into this and twisting it a little too much, but think about it for a second: If he can't see how both worlds have good, then maybe he can see how there is just as much bad in one as there is in the other.