What was behind Michael Myers eyes?

Answer 1

To quote Doctor Sam Loomis:

"I met him, fifteen years ago. I was told there was nothing left. No reason, no conscience, no understanding; even the most rudimentary sense of life or death, good or evil, right or wrong. I met this six-year-old child, with this blank, pale, emotionless face and, the blackest eyes... the *devil's* eyes. I spent eight years trying to reach him, and then another seven trying to keep him locked up because I realized what was living behind that boy's eyes was purely and simply... *evil*."