Usually, its “I want to be famous” as tragic as that sounds.
In my highschool there was a kid who was prevented from knifing a few kids at school he wanted to kill. He constantly read books and watched TV shows about serial killers and terrorists, idolizing them in a creepy way. Anyone who did massive violence he idolized.
I’m glad I don’t live in the States. He could have easily gotten automatic weapons there.