I'm just a first-year law student, but on the contrary I think that it must take a tremendous amount of morals to be a criminal lawyer. I mean, one must have a rock-solid faith in the system to put people's lives in its hands every day. Sure, there are some sleazy punks who will take any client for a buck, but the vast majority of DA's and Public Defenders believe that our criminal justice system is the best method, albeit an imperfect one, for finding the truth. What can be more moral than that?
I say follow your heart and you can't go wrong.