I can't even fathom what would cause girls to be fans of a mass murderer but I've seen it happen with so many of them. There were so many girls who became fans of the Dark Knight Rises shooter that they collectively refer to themselves as "Holmies". I've seen some who claim James Holmes was framed, while others try to justify his actions by saying a cruel society forced his hand. I remember reading an article a long time ago that said he had naked photos of girls who sent him pictures taped to the walls inside his cell. (I was watching HLN earlier and they were talking about the letters and money that had been sent to NC, but they said they are keeping those types of letters from him and he doesn't even know that the letters exist. The only mail being delivered to NK are those of a religious nature.)
I think a lot of the girls who join these types of "fan clubs" just do it because they desperately need to feel "accepted" by any group (& it doesn't matter what kind it is) and I think some probably post these declarations of love on social media for the shock value to garner attention.
There are even fans of the Columbine shooters. If you search the name Eric Harris on YouTube, you find a ton of "tribute" videos with montages of pictures and the girls gush about how hot and "bad



" they think he was. It's disgusting.