The kind of girl/woman who does that is, or likes to see herself as, an outsider who just doesn't fit in anywhere, and they think because they don't fit in because they're weird or socially inept, they can genuinely relate to the kind of guy that does that kind of stuff. They like to see each other as sort of doomed lovers or whatever, at war with the world but totally in love with one another. Also a lot of the serial killers and mass shooters that do wind up with a cult following are generally conventionally attractive young men anywhere from like 18 to their early 30's. I don't know about any of you, but I've never seen a bunch of Dennis Rader or John Wayne Gacy fangirls (although with my luck I've just jinxed myself and now I will).

Source; this was me and a few of my friends were like that with the Columbine shooters from around 13-15 years old. I'm so glad I grew up.