I think when he is looking down from upstairs at Mark hugging his mother.
Actually it started when Mark stood up to Henry telling him:
"My mom said she'd always be with me. She chose your mom as a way of coming back, but I guess it would be hard for you to understand that, but it's true. She's my mother now."

I say pretty much when he starting seeing that he is evil

This message has been deleted.

It's a combination of things. I think what really got him going was the day of the dummy. Not only did Mark not agree with what he did and told him off but Mark also took Henry's sister's side and stood up for her. This I believe is what really made Henry want to get rid of his sister right away. I mean I believe at some point he was eventually going to try and kill her in the near future but Mark really pushed Henry over the edge. He hated Mark but was furious at his sister for getting Mark to take her side.
The scary clown doll is hiding under my bed.