If you have ever participated in organized combat sports; boxing, MMA, what have you; then you will understand that there are many ways to fight an opponent, and that it is essential to fight your opponent on your own terms. That is; to fight them in accordance with your own strategy to methodically neutralize their strengths and maximize their weaknesses; rather than according to your reactions to their strategy. Prior to the fight, and during it, the opponent will try to provoke you, to anger you, to make you abandon the discipline of strategy and to instead fight him in a blind rage. This is a tactic used particularly when you and your opponent are evenly matched, or, even more so, when your opponent is stronger than you. If you are stronger than your opponent, you can afford to fight in a state of rage, but if you are equal in strength, or weaker in strength, you cannot afford to fight emotionally; so a clever opponent will try to provoke you because he knows you can only win through strategy.
This applies on a one-on-one confrontation, and it applies on larger scale conflicts. This is the basic explanation for atrocities committed against vulnerable populations.