This boyfriend sounds as if he's an asexual. The lack of attraction but willingness to go through such lengths to please you seems somewhat typical of an asexual.
If this is the case then sex to him doesn't mean the same as or is as important as sex is for you.
He can not always give you what you want so he doesn't mind you getting sexual pleasure from someone else but he does mind if you become emotionally intimate and attached to another because the emotional and romantic aspects of your relationship means more to him.
In short he may view anyone else you have sex with as living sex toys.
As for the detesting violence but supporting a death sentence he may see it as a way of punishing those who has done violent things. It's like a means to an end.
But these are merely assumptions it's best to talk this over with him.