Re: Shooter games
In general to get someone to kill other people as a matter of course they need to believe the victims are "other". Even when someone is mentally ill - that perception is probably still necessary to override empathy.
That often means a definition as "subhuman" - but otherwise could be a perception of some "threat".
To put other people into such categories is down to the culture of a society in promoting "them against us" polarised ways of thinking across the board.
Those who quote the biblical Commandment "Thou shalt not kill" - should remember that it was meant to apply only to those within a small society who conformed to certain dogma. People who did not conform - or were from rival societies - were explicitly defined as fair game or even compulsory targets.