"Psychotic" also works here, but for a different reason. While a psychopath may be able to perceive, but unable to empathize with, a person, someone in the depths of psychotic delusion may be unable to perceive a fellow human being. It is perhaps not nearly as common "in the wild", but it does occur (the killing of Tim McLean by Vince Weiguang Li being the example that comes most easily to mind). In fact, generating a sort of "target psychosis" has been the aim of most modern infantry training for some time, since teaching people to see, recognize and react to targets is an awful lot easier than training them to kill humans, even under severe threat. (Human-shaped targets were introduced to training after it was discovered that most infanteers never actually fired an aimed shot at an opponent in either of the World Wars.) Similarly, you can sniff war on the horizon when the machinery used to dehumanize the enemy is started up.
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.
You're right about the percentage increase. This is for two reasons - first, because most of the close-range shooting on the ground was done by special forces who're trainined to overcome that resistance. Second, because many of the deaths are from bombs - either shelling from ships at sea, or from aircraft and UAVs. Those last three remove the personal element, and that makes "pushing the button" really easy and guilt-free. We as humans who pride ourselves on empathy can be pretty savage :-(
I could be mis-remembering the article in question, but I think the percentage increase referred to soldiers who fired their weapon (e.g. assault rifle or sidearm) directly at an enemy they could see - meaning that we've become more desensitised to violence over time.
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.
> meaning that we've become more desensitised to violence over time.
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.
