Until Firebomb, Sark has been a bit one-dimensional. He was polite, smiled often, and killed easily. He's different in “Firebomb”. We’ve got the first hints from his questioning behavior that he is not totally aligned with Sloane. He’s trying to reason out why Sloane did what he did, so the end plan that Sloane is interested in has not been fully revealed to him. He definitely seemed reluctant to kill civilians with the neutron bomb. Yes, he’s killed before but killing K Directorate and the people in Hong Kong would be killing other bad guys. Yes, he had the evil dentist torture Will, but he may well have given instructions that Will was not to be killed, but merely be given a little excess dentistry. Sark now seems like he might not be the bad guy he once seemed.