What Caused The Killer to Snap(Spoilers)

What was the catalyst or event that caused the killer to snap and start killing? I'm almost positive that he had not been planning this his whole life. Or was he?
Oh, I'm watching now and I think I've got it. I think it was the mute boy who was being treated, having similarities to Michael. Am I right or was it something else?

"Do All Things For God's Glory"-1 Corinthians 10:31
I try doing this with my posts