A motive for a heinous crime?

Here's a quote from the movie Batman (1989) where the Joker says this:

Joker: I now do what other people only dream. I make art until someone dies. See? I am the world's first fully functioning homicidal artist.

If somebody who's a nobody wants to make a name for himself, he can copycat that character.  That's what some of these mass murderers must be doing.  Making a name for themselves by doing some performance "art", like the Joker in the Batman movies.