This thread is so long I'm just jumping in here. Forgive me if some of this has already been said.
First, I can't imagine how anyone could do this. I just don't have the ability to fathom that level of mental illness
I've heard that the classroom he targeted was his mother's. I don't know if she was killed before school and was a regular teacher or what. If it's true he targeted his mother's classroom, he was enraged at his mother. Not only did he want to kill her, but he wanted to kill the children she loved. Playing amateur psychiatrist here. I think he felt like she cared more about her students than she did about him. I think it was something regarding this that set him off. JMO
The next thing I'm going to say may be offensive to some, but it's how I feel. I have zero sympathy for his mother. She didn't deserve to die, but she purchased her own, and the children's deaths. IMO, there is absolutely no reason someone needs to own an assault rifle. They are used for "target" practice mostly. She purchased them and did so legally. IMO, a gun that is legal is only legal until it's used illegally.
There are 26 people from that school who are dead now because of that gun. You can say well he would have killed his mother with a knife. I agree. However, I doubt he would have been able to kill 26 people in the school with a knife before he was stopped. JMO
One more thing. I support the second amendment and a persons right to own a weapon. However, there should be much stricter limits to owning one and there are a lot of types that should be illegal.
In order to drive a car, I have to take a written test, then I have to have an official test me on the road to be sure I know how to handle a car. I'd love to be able to take a Formula 1 car on the highway, but if everybody was allowed to own one then there'd be a lot of traffic accidents and deaths.
In order to get a gun, please correct me if I'm wrong, you have to pass a written test only. You don't have to go to a police firearms range and prove you know how to safely use one and can shoot fairly straight. I believe the same rules that apply to getting a drivers license should apply to getting a gun.
All of that is JMO