What made Loomis realize that Michael was just evil?

When you study this kind of medicine as Dr.Loomis did. I guess you can just sense in people what and who they are. And I guess with Michael he could see no remorse. no emotions...nothing. And in his mind considered that to be evil. You can come to such a conclusion by studying them and interacting or at least trying to interact. but Loomis could see more .. he was probably only studying Michael and no other patient .. so he saw things others didn't. when you are with someone so much, you can pick up on things that no one else notices. and he saw evil in Michael.

"You gotta live,live,live...life is a banquet and most poor suckers are starving to death!"

Possibly the lack of progress, we can't be sure what exactly Michael was doing there.

I like to think that it was simply Michael's lack of response or interaction with him that caused Loomis to think there was nothing to him except his evil