I'm assuming that the mom quit working when her son dropped out and got his GED. That would make sense, he would have been in his teens and not old enough or responsible enough to be left alone so when he quit school, she had to quit working also.
This probably threw them into a cycle where they were spending excessive amounts of time together with no end in sight, after all, he wasn't going to grow up, get married, go off to college. His brother would, the kids his mom taught would, his former classmates would, but not Adam. He was probably going to remain the troubled person he had always been, only in an adult body, but without adult privileges (he didn't have a driver's license that I have heard of) and both Adam and his mom realized this.
I am sure this was difficult on both of them. She was going to have to care for him for life and at age 20, he was realizing that age 21 was approaching but he was not going to have a normal life like everyone else. No job, no wife and kids, no college, etc. Just the same life, mom and son, until one of them died.
She was stuck with him but with limited powers because he was of legal age and intelligent and didn't have a criminal history. She couldn't control him as easily as she once had, he had grown to adult size. He was stuck with her because he couldn't survive out there on his own. There was probably a power struggle that neither of them talked about.
Sad but this situation is going on in many other households as well. There are many other time bombs like Adam out there. And while they wait for their time to "go off", they amuse themselves with video games and movies full of violence and carnage. Mind you, I am not blaming video games or movies.
I can't find anyone to blame but him. We can lock up school, outlaw guns and take all kinds of precautions but in the end, if someone like him is determined to cause mass destruction as he exits a world that he hates, he will find a way to do just that, in one way or another.