Slayers are stronger than humans but their powers are rather puny compared with those the X-men have. All you'd really need to control one is a good shot so if I were the government I'd be far more concerned about magic users than Slayers.

Agree, but, unlike mutants, slayers may disappear in the crowd. They may impose global terror. They may create new cult. In any case, in RL the whole social system would be completely destabilized and, I suppose that "a good shot" - or rather "many good shots" would be the most probable scenario.