I suppose it's no surprise that we feel the need to dehumanize the people we hurt--before, during, or after the hurting occurs. But it always comes as a surprise. In psychology it's known as cognitive dissonance. It's the idea that it feels stressful and painful for us to hold two contradictory ideas at the same time (like the idea that we're kind people and the idea that we've just destroyed someone). And so to ease the pain we create illusory ways to justify our contradictory behavior.