"And you always have to be so difficult."

He has First Aid outgunned by a mile. He clearly has the upper hand here, and there's really nothing stopping him from taking an eye for an eye. Except that he can't quite bring himself to fire -- which is odd, since he already went to the whole trouble of holding First Aid at chainsaw-point and all.

It's the vague contemplation of what he's doing and what he should be doing that keeps his cannons at bay.