The recent Florida mass murderer was known to be abusive to his ex-girlfriend.

EDIT: I like this bit from that article: "...I believe in boys. I believe in my son. Sometimes, though, I see him, 16 years old, swallowing his frustration, burying his worry, stomping up the stairs without telling us what’s wrong, and I want to show him what it looks like to be vulnerable and open but I can’t. Because I was a boy once, too..." But I think he incorrectly attributed that behaviour of his son to simply being a "boy" when "being a teenager" is the real reason. All teenagers go through that behaviour because, you know, they're going through puberty. END EDIT.

Here's another perspective from Bill Maher, from 2015, in his response to the college campus shooting in Oregon of that year. Maher says: "...why young men with guns use them in mass shootings..." He lists the obvious and usual reasons people give: "video games, mental health, bad parents, lack of prayer, etc..."

Then he says: "...Then we have to get real on what it really is: THEY CAN'T GET LAID." He presents a good argument on this.

Jump to about 2 minutes of this video to skip Maher's "New Rules" jokes.