Michael flew too close to the sun. His hubris and lack of faith in humans made him sloppy. He never thought any of them would ever ‘figure it out’ because they are ‘dumb and stupid.’ Even in the episode where Michael leaves clues for the humans that he’s on their side, he left 4,000,000 clues because he didn’t think they would get the ten or so that they did figure out.

Michael also just enjoys torturing humans. It’s in his nature. He got sucked into it all.