If Tony was going to have this much of a reaction to killing someone, then he had no business sticking with Kaz. Because Kaz had no conscience to feel guilty about as it stands. He was infamous for accepting any job regardless of danger or the ferocity of violence called for. Kaz himself would say he only has two rules: no kids and no slaving. Anything else up to and including murder was fair game. But Kaz would also admit he had no intention of killing anyone else, but he said nothing to Tony's declaration because Kaz also had no intention of using less than deadly force if it was self-defense. For the moment he was letting Tony and the rifle go first down the stairs.