Louisiana mortician Rick Stanton was a pioneer in selling murder doodles and organized the first art exhibitions of famous killers' work. Stanton is considered by many to be the originator of the collectible craze, saying the murderers' art is, " ... the stuff where the feelings are everything, where the person is everything ... They're not concerned with trends, or with sales in the art world."
Another piece by Charles Manson
It should be noted that serial killers probably aren't concerned with sales mostly because of laws that prevent them from making any money. In 1977, a little doggy told David Berkowitz he could make a truckload of cash from a tell-all book, and publishers came running. These events lead to the formation of "Son of Sam laws," which prohibit a felon from profiting off their crimes, and preventing mediocre art school students from going on a killing spree.
Also, a brief analysis of the works by the killers themselves shows no correlation between depravity and talent. It turns out a shitty painter will remain shitty even after disemboweling a hobo. For instance:
Above is an orgy skull dreamed up by Gacy when he got tired of painting clowns. Yes, it's a skull made of dongs. This is way scary, and not laughable or pathetic in the least.
This depiction of a mermaid tickling a shark was miscarried into the world by killer Charles Ng, whose body count totaled over 25 people. He felt the compulsion to bring more suffering into the world and designed the poster for Jaws vs. SPLASH!.