Not posessed, just fucked up in the head. And his parents more than likely played crucial role in that so in a way, they got what was comming for them.
There was a guy from my high school - fat, curly long hair, quite smart, but nothing exceptional for my school. It was "elite" high school with dormitory where kids who were out of town lived during semester. I didn't know him very well, my best friend at the time knew him better - he played counter strike with him on LAN in the dorm. He was a typical childish fatso that was never anything out of the ordinary.
When he graduated he started studying in another city, even more distant from his home town. That alone should tell you something - such urge from the young age to run away from your home tells you it must have been dysfunctional. Surprise, surprise, guy turned out to be gay. He found some old gay lover and decided to tell his parents all about it. They weren't so happy about their son being fucked by some old dude.
He bought axe in city where he studied and during winter break or whatever, came back home with it. When his parents fell asleep he butchered them with that axe and then called the cops.
His grandparents defended him until the very end. You see, he was the only child in this family. They claimed it must have been someone else even though their grandson admitted the killings. It was a small town where everyone knew everyone. Despite that, no one has ever said anything bad about this family before or after the incident. They simply had no idea.
There are no ways to prevent such events. Psychology is a bad joke. Dozen of pro psychologists took care of Elliot Rodger and they haven't seen what was comming. They are fucking AWESOME at explaining why someone did something after the fact, especially in front of the cameras so they can shine with their "intellect" and "professional opinions". Situations like these are very rare, number of casualties is very low and you can't do much to prevent without giving up our freedom. It's like getting hit by a car. Shit happens, move on.