What do many mass shooters have in common? A history of domestic violence.
Among other things, there is a correlation between Domestic Violence, among the ones with Domestic Partners. However, a lot of them are increasingly from communities like the MGTOW (Men Going Their Own Way) and Incels. So, outside the statistics from those that are married, or trying to maintain a relationship, the singles also show a pattern of mysogyne, and blaming women for their problems.
Also, every single one (Including small groups, like the Columbine boys) show a unmistakable pattern of Attention Seeking/Cries for Help. While we're looking at these things, we might want to consider which symptoms will actually help us stop them, before they get their hands on guns. Disarming guys that beat their wives only really works on the ones with wives to beat. (Assuming it doesn't trigger their pesecution complex, and "Cold dead hands" delusions of grandeur, knowing that they know other gun owners, and can easily get more illegally. Or just make Pipe Bombs.)
We ignored them, that's part of their motivation. They hate being ignored, and go to great lengths to get our attention. Ultimately mass murder/suicide when all else fails. I respectfully suggest we start paying attention, before they claim it over a pile of school children's bodies.