It provides a logical conclusion for me. Chris is a sexually obsessed autist, and his relationship with Barb always was weird at the least. I never expected Chris to do this, but I'm also not terribly surprised.
As a fellow autist with legal troubles myself, Chris is only unique because he's had his entire life plastered over the internet. I've seen many autists who never received proper help, some of which have ended up homeless and waiting to die. Others that cleaned up enough and learned to shut up so they could NEET n' chill in subsidized housing for the rest of their lives like myself.
It's tough, especially for old parents like Bob that had no idea what they got themselves into. Chris was screaming for help with his actions, but people involved didn't know what to do. Some like Mary Lee Walsh tried, but there's only so much power she had. And when the plan all along seems to be Chris living with his Mom forever, and eventually being her caretaker... well, it was all a disaster waiting to happen, tbh.