In other words, he was very likely mentally ill. The deeply disturbing content certainly supports this hypothesis. He was also filled with rage, pain, and likely self-hatred; hard to imagine another motive for the desire to blow your brains out in the most public way possible.

My sister was filled with rage, pain, and self-hatred. Had our courses in life been reversed, had I been born on the spectrum, been teased and made fun of my whole life, never had a true friend, had no possible prospect of a romantic relationship or family, I would have been filled with rage, pain, and self-hatred.

For the most part she turned those emotions inward, followed hugely unhealthy habits, and died of cancer at a too young age. It's certainly possible that I would have turned the feelings outward, and given my early training in firearm use, it's possible I would have lashed out like this shooter.

Toxic standards of masculinity certainly played a part in his actions, but so did the hyper-competitive cultural norms to which both women and men hew. Boys and girls are both made targets by their peers. Most if not all children understand the zero-sum nature of popularity, success, respect, and acceptance, fight sometimes viciously to not lose, internalize at an early age what their prospects are, and feel justifiable hurt, rage, and self-hatred if they judge that they will lose this competition.

It's easy to blame amorphous and vague cultural concepts for the very specific harms inflicted on children forced to compete for a limited number of "successful" high status positions, injuries that lead to rage, self harm, and the harm of others.