Canon authors should not take part in fanon 'ship wars. The resulting "my pairing is pastede on yey" appeals to nearly no one.
If you write a character three-dimensionally, someone out there is going to like them, or like who they once were, or like who they could have been.
If you base what you fondly believe to be a cardboard character on an actual person you once knew, you're going to write them three-dimensionally in spite of yourself.
Believe it or not, some people find morally ambiguous characters hot.
On the other hand, some people do not find it at all attractive when characters who are supposed to be good do morally wrong things and then claim that they're still the good guys. (Hint: do you think George W. Bush is attractive?) You can get away with it when they know they were morally wrong, even if they do feel that they were justified. Justified != right, and don't you ever forget it. You write Snape so very well, you should know these things.
If you think I should ever like James Potter again, you have another think coming.
Does "No woman in her right mind would ever sleep with Snape" mean that he's really gay and that Potter is the only one who can understand his tortured angst at striking the final blow to Dumbledore, and after Ginny dies a tragic redheaded death, Potter and Snape hook up? Please tell me I'm right. (If I'm not actually right, I'll just go and write fanfic, and I'll probably write it better than you could, which will probably piss you off to no end.)
Morally Ambiguous Love,
a smirking fangirl who pre-ordered from an independent bookstore and then rode four hours on a bus to pick up the blasted brick