Later, after Mr. Rodger’s 140-page manifesto was released — outlining his fury over still being a “kissless virgin” — his name became synonymous on misogynist forums with revenge on women who reject men. Chris Harper-Mercer, who shot and killed nine people at Umpqua Community College in Oregon in 2015, mentioned Mr. Rodger by name in a manifesto he wrote in which he complained about being 26 years old with “no girlfriend, a virgin.”

And now, in the aftermath of the attack in Toronto, men on incel communities are hailing the killer as a “new saint,” with commenters changing their avatars to Mr. Minassian’s picture in tribute.

Feminists have been warning against these online hate groups and their propensity for real-life violence for over a decade. I know because I’m one of the people who has been issuing increasingly dire warnings. After I started a feminist blog in 2004, I became a target of men’s-rights groups who were angry with women about everything from custody battles to the false notion that most women lie about rape. In 2011, I had to flee my house with my young daughter on the advice of law enforcement, because one of these groups put me on a “registry” of women to target.

I was far from the only one. In 2014, a gaming award ceremony set to honor the feminist critic Anita Sarkeesian received a bomb threat; an anonymous harasser threatened to detonate a device unless her award was rescinded. Before Milo Yiannopoulos was a well-known alt-right figure, feminists knew him as one of the primary architects of Gamergate, a movement of young men who harassed and threatened women in the videogaming industry. Two fans of Mr. Yiannopoulos were charged with shooting a protester outside of one of his speeches.

Part of the problem is that American culture still largely sees men’s sexism as something innate rather than deviant. And in a world where sexism is deemed natural, the misogynist tendencies of mass shooters become afterthoughts rather than predictable and stark warnings.

The truth is that in addition to not protecting women, we are failing boys: failing to raise them to believe they can be men without inflicting pain on others, failing to teach them that they are not entitled to women’s sexual attention and failing to allow them an outlet for understandable human fear and foibles that will not label them “weak” or unworthy.

Not every attack is preventable, but the misogyny that drives them is. To stop all of this, we must trust women when they point out that receiving streams of death threats on Twitter is not normal and that online communities strategizing about how to rape women are much more than just idle chatter. There is no reason another massacre should happen.