“So people who watch/read material with murder in it then have a desire to commit murder?”
Yes, that might very well be true - statistically speaking.
Could our murder-themed entertainment have any effect of the rise in violent crime over the past 100 years?
Might people that read about adultery might be more inclined to cheat on their spouse?
Might young people that read of the glories of drug use in the 60 be more inclined to try drugs?