In experiments modeled around conflict, game theory has established that the most successful strategy is tit for tat: never attack, retaliate in kind. Those who initiate conflict invite a forceful response. Those who do not retaliate when attacked invite further aggression. Both initiations of violence and passive responses reduce survival rate on average. As such, tit for tat is a mathematically consistent selective pressure. The retaliation instinct is a biological adaptation to that pressure. This is why every human culture has some concept of the right to self defense, and aggressive behavior is condemned.
This instinct is amplified at a societal level. When attacked by an outside force, human groups tend to close ranks and rally behind the leader. Limits on power are brushed aside. War becomes a moral imperative; calls for peace tantamount to treason. This response can be triggered artificially.