'Why' is a mystery (and one not likely to be solved, since I think Zz'dtri is probably not going to survive this fight, let alone this book), but the fact that he's doing it in the first place
shouldn't be in question.
As theNater points out, it's not as if tailoring his build to fight V is worthless against other wizards. And saying that V isn't particularly powerful seems questionable, seeing as she's at least fifteenth level - that's not a common thing in a lot of settings, and doesn't seem to be here.
Edit: On second thought, isn't getting him dragged off by lawyers a decent reason to hold a grudge? I'm not sure if anything more complicated then that is needed, especially since drow have been traditionally characterized as putting a high value on revenge.