Both of those are in response to him already killing people and being an obvious issue by the final act of the movie. I doubt mom would have done anything if he hadn't killed dad, and dad only did so because he killed uncle (I think, one of the bodies they found).

But a lot of the prior people are just either dumb kids doing normal kid bullying or normal adults either trying to discipline him or being annoying busybodies. The kind of thing that feels really overwhelmingly bad as a kid and makes you want to "get even" because you don't understand it.

So he uses power he shouldn't have to act on unrestrained kid emotions, and it goes wildly bad. Which is less "omg he is so evil" and more "kids don't react well to having zero limitations."