Which characters show us that they have principles? What are they willing to do to stick by their principles?

Ken is Ray's mentor and fellow hitman. They both work for Harry. Ken escorts Ray to Bruges and is then given the order to kill Ray by Harry. Ken is ready to fulfill the order when he sees Ray attempting to take his own life and he stops the younger man from doing so. Ken sees that Ray is a man who needs a second chance to redeem himself and believes him to be a good man who's done wrong. He believes it so much that he gives his life to protect Ray.

Harry is the superior to Ken and Ray. He's ordered Ray to Bruges and eventually order Ken to kill Ray. He does so as Ray has accidentally killed a young boy during a hit, an act that he believes is unforgivable. Once Ken rejects his order to kill Ray, Harry comes to Bruges to do it and take out Ken. He eventually chases Ray through the streets and accidentally kills Jimmy, a dwarf, and believes he has killed a child. Believing so much in his principles he shoots himself in the head.