Well, first off, it is a known fact that he hates people with powers since his sister was killed by one. Second, it known that he joined S.O.T. since he felt that he failed to protect his sister, so he cared about her. As for why he terminates more dangerous powered people, it is because he feels the need to protect people without powers from being harmed like his sister was. That is one angle I am think of going for.