Actually, Leonard is a grade A asshole. All he does is stay hunched over making hate-filled, snide remarks about everyone around him if they don't have a pair of tits. Galecki himself doesn't come across much better in interviews either, at least to me.
I'm not sure we're watching the same show, because I don't see that at all. Granted Leonard is far from my favorite character, but he's no asshole. I know a few 'grade A assholes' and even consider one of them my friend, and Leonard is nothing like them. What I think gets mistaken for snide or hate filled comments, is just friends giving each other a hard time. I do it to my friends and they do it right back.
I don't know. That stunt he pulled on Raj last night--"Tell the new girl all about your work."--was pretty sadistic. He pretty much set Raj up to humiliate himself, just for the fun of torturing him a little.