Brian's a murderer and heartless about what he does, but he's not cruel, surprisingly enough. He thinks Birkin's kind of nutso for even wanting to control the world. He kills people out of a psychological to need to feel "in control" and a bunch of other developmental psych theory, but long story short as far as psycho killers go, in his heart of hearts? He's not that bad.

I could give you a whole essay on the root of acrotomophilia BUT I'LL SPARE YOU ANNND he'll be sympathetic to Stein's dilemma. He has a moral compass, he just... ignores it.