While kisama is often translated "bastard", there's a really good reason not to use it that way here:
Momo doesn't have parents, or indeed any family. At best, she's an orphan, but there's a very good chance she's a literal bastard. Using that translation would make Shamiko seem to be making personal insults that she never would use.
I'm not sure "asshole" is the best choice, but if the intent is to convey "you jerk" with an extra emphasis on the disrespect, to the level of vulgarity, it does fit.