After looking for info on Wikipedia I only found one brief mention of school shooter fandoms in the “Columbine effect” page:

Since the advent of online social media, a fandom for shooters Harris and Klebold has had a documented presence on social media sites, especially Tumblr.[86] Fans of Harris and Klebold refer to themselves as “Columbiners.”[87] An article published in 2015 in the Journal of Transformative Works, a scholarly journal which focuses on the sociology of fandoms, noted that Columbiners were not fundamentally functionally different from more mainstream fandoms. Columbiners create fan art and fan fiction, and have a scholarly interest in the shooting.[73]

The articles they link to are:

1. How Tumblr’s True Crime Fandom Reacted to the Escape of a School Shooter

2. The Killer Crush: The Horror Of Teen Girls, From Columbiners To Beliebers

3. Fans of Columbine shooters Eric Harris and Dylan Klebold

I also found another interesting study on this titled “Glamorizing rampage online: School shooting fan communities on YouTube”.

They examined the network structure and social and psychological aspects of YouTube communities centered around school shootings, finding that they form a small-world network characterized by high clustering and low density, i.e. loosely-tied subgroups of fans. They argue that this network structure promotes the dissemination of hate ideology and justifications for mass violence, and also has the potential for anti-social behavior, which is consistent with previous analyses proposed by scholars. They also note that the people in these fandoms often talk about depression and other mental health problems, and argue that providing consultation to these people could help them leave these communities and reduce the spread of hateful rhetoric - however they also argue that the current reporting system may be insufficient to deal with these groups.

Article 1 also gives some anecdotal evidence of how some of these people justify their fandom by minimizing its ties to violence:

Shotsofdylan, the same blog that posted the bit of Dylan/T.J. fanfiction above, wrote:

“People who actually think we support murderers and condone their actions are fucking idiots. If you bothered to talk to most of us, we don’t condone anything bad they ever did. And most of us are intelligent, forgiving, compassionate people who learned a lot about humanity and life through the murderers we’re interested in. Don’t talk shit about us before you get to know us. Thanks.”