Not us. The FBI, the school board, the parents, etc.
They're at fault for not protecting their own schools.
Trust me, even if the school administrators held a meeting about a kid making threats, you're going to have a lot of liberal parents suggesting that he is a misguided kid who just needs an adult to make a positive impact in his life. Most of them wouldn't have voted to remove him from the school.
Reports came out, including from his own aunt, that he had a very troubled upbringing. Looks like his mom had a long history with the police and his dad might have been a wife-beater. Obviously, the dysfunction at home meant the parents used the school as daycare so they could soothe their own personal problems rather than raise the kid right.
Schools aren't allowed to suggest homeschooling to parents. The parents have to come up with that all on their own. Most parents won't pull out their troubled child. Heck, in fact, they throw the kid on the school to parent.