Nitpick: You mean "psychopathic", not "psychotic". People with a psychotic illness are more likely to be victims of violence than the perpetrators of violence, and are much more likely to harm themselves than to harm another person.
http://www.historynet.com/men-against-fire-how-many-soldiers...
I read somewhere (citation needed!) that the percentage of soldiers who deliberately fire their weapon at the enemy has increased from WWI to the Iraq conflict, which some put down to violence in movies and video games. After watching excellent Iraqi conflict documentaries like Restrepo and Armadillo (Danish movie) and seeing how soldiers react to conflict, there could be merit to that argument.
I also recall that snipers are trained by shooting watermelons - the impact of a high velocity round on a watermelon is not too dissimilar to an impact on a human head. Snipers see the carnage in all its gory detail because of their magnified scope so it's necessary to desensitise them in training.
Edit: slo-mo watermelon getting shot: http://www.youtube.com/watch?v=iO3XsZpOiBA
And yet the rate of violent crime continues to go down, bear-baiting and dogfights have gone from national pass-times to illegal aberrations, and dueling is pretty much extinct.
There's a book that describes the effect in war. The basic premise is that it's easier to kill people by dropping a bomb on them from an airplane than it is to shoot them with a rifle on the battlefield.
It's because in the latter you see your victim. And immediately relate to that victim [edit: 1]. Which makes shooting him or her exceedingly difficult.
It's an interesting parallel.
The book is called On Killing, by US Lt. Col. Dave Grossman.
[1] Unless the shooter is psychotic.