Perp was a heavy pot smoker and on numerous occasions before this, was friendly to the child, often gave him toys to play with, etc.. (per interviews on the local Chicago news affiliates.)
Something snapped with this guy. One of the TV Stations (IIRC, WGN Channel 9) had a reporter stand on a latter in the boys back yard and take a shot of all the pot plants growing in the perps yard. Paranoia is one of the side effects from heavy pot smoking, as I've read.
(NOT a pot smoker here.)
Many years ago, I used to smoke a lot of pot, and I can tell you that without a doubt paranoia can certainly be a factor. I haven’t smoked any in probably 40 years, and it sure made me paranoid. That’s primarily why I quit. I didn’t like what it was doing to me. I’m better off for it.