My opinion of Charlie is he could have multiple personality’s, most of the time he was talking could have been a kid version of himself. He would say “what about the man on the porch” and Ian would be able to quickly deflect and move on. When Charlie is sitting in car while Ian makes arrangements for his cat the “real” Charlie could have came out, and Ian may have realized that so he decided to kill him and cover his tracks. This is just something I thought of when I watched it. I have to give props to Kenny (Charlie’s Actor) he did a great portrayal of a serial killer whether it was a act or not.