I feel sorry for the kid, myself. Been there, done that...minus the killing of the tormentors. Dad had a gun. So did mom. But I never went so far as to do what the kid did. However...from reports, and intuition of my own, I think the kid was maybe bullied or abused at home, too. He had no place to turn. Hence the "badass" he tried to portray on social media and even there, he didn't have many friends. To end the torment...he wanted to die and take them with him. Maybe he was afraid to tell his dad. Did he even have a mom? Maybe he did tell them and he was laughed at by them too, or called names. I told my mom when it was happening to me. She did nothing. I didn't tell my dad...until I finally had enough and ran home a sobbing mess. He didn't call me a chickenshit, or make me feel worse. He is the one that taught me to fight. He said "never start a fight, honey. But protect yourself. And this is how". He moved all the furniture from the living room to the side and he gave me many lessons on how to fight, how to protect myself, where to punch/hit/grab. After that...I went back to school (from being kept home by Dad for a week), and had more self confidence in myself. The bullies found out soon enough I was not as weak as they thought I was...and I realized it too.
Anyway....that kid had enough. ALL of them that did school shootings had enough. You can blame guns, parents, jiffy peanut butter if you want. But the ROOT is the school itself and the bullies that get away with their shit.