It wasnt accidental. SLIs are evil. That was in him and he knew it. Deluded himself he was normal. A lot of the normal ones are repressed evil. I have angered SLIs before. They kill so easily. ILIs are a little more controlled. His psyche was such that it allowed him to be like that. You can't fully will what you will. They kill all the ones who stand out especially then there's only the "normal" ones left.
They are only human when they get threatened. Their victim is not only human. It's only a human.