One scene that i remember:

The boy with the mental illness brings a girl to his home to use her in an art piece. He steps out of the room for a minute, and she goes through his old sketches. When we walks in and catches her snooping, we flips out and i believe he kills her.

The only other scene i remember(which may or may not be the same movie):

There was a male sitting on a couch or a bed, when the boy enters the room. He sits on top his lap and begins to squeeze his nose while holding his mouth to suffocate him. When the guy stops breathing and becomes unconscious, the boy panics and gives him cpr to bring him back.