"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.