It is not brainwashing, it is just the releasing of a primeval human desire to kill.
The only reason people do not habitually kill each other all the time are the social and moral constraints that come from civilisation - the adherance to a particular group or culture in which they live. Once those social boundaries are relaxed or removed humans revert to primeval mode and the urge toward violence is high...which is why throughout history victorious soldiers often kill, rape and pillage after a battle - which they would never do in their domestic setting - simply because they can.
It is uncommon anong Western armies, but not unheard of. It was extremely common as the Russian army went through Germany at the end of WW2, but possibly in retaliation for what Germans did on the way into Russia. It happens frequently, now, in African conflicts - women raped by soldiers and maimed or killed afterwards - pre teen children killing as if they were born to do it.
The depravity of humanity without social constraints is scarily awesome to behold.