My wennerae hasn't killed anything in it's tank (four hermits, four turbos, four emeralds) that I've seen. The only thing it's ever done is try and use the smaller hermit's shell to decorate his doorstep with, and then he gets kind of annoyed when it walks off.
I target feed him frozen mysis shrimp on a pike, and I guess that's enough for him. That said, he will punch the emerald crabs if they bother him. He's taken off a couple legs on one, but nothing lasting.
So YMMV, each one has their own personality.