So, when you're dealing with someone that you call "Walking Calamity" there's really only four rational behaviours you can have.
1) You control him if you can manage to. In that case, Profit! Harness his powers for your objectives, whatever they are
2) You kill him to remove the problem. This is also a very rational thing to do if you feel that things could turn out very bad
3) You observe and study him. Often before choosing one the the other options.
4) If you cannot do any of the above, you panic and try to lie to him, convince him not to use his powers. This could be the case here, because they're not telling him what they know.
I wonder which of these situations we are in. I'm rather intrigued.
And I'm really, really curious to know what the hell is the deal with his sister. Has she been mind controlled? Has he been framed? Or she's just unstable/insane?
For such a silly manga, this has me rather invested I have to say....