Life isn't that simplistically black and white. Just because a civilization does bad things doesn't mean they're pure irredeemable evil. Japan launched a sneak attack on Pearl Harbor, but now they're one of America's staunchest political allies and the source of a great deal of our beloved pop culture. The Klingons nearly conquered the Federation in the 2250s, but became their allies a century later. The Vulcans and Andorians were at war for centuries but ended up co-founding the Federation.
The thing is, people often strike out because they feel threatened and afraid. And the people they strike out at often mistake that for aggression. So sometimes both sides in a conflict think they're the ones defending against the other side's aggression. That's basically what happened in the Cold War, as I learned in college. Russia had a long, long history of being invaded and conquered, and the Soviet Union was basically surrounded on all sides by enemies or rivals (counting the US across the North Pole). So the USSR felt it had to build up its military and nuclear arsenal to defend itself against another invasion. But Europe and the United States had a history of engaging in imperialism and conquest, so we saw the USSR's military buildup and assumed it was intended as aggressive, so we built up our arsenal and nuclear stockpile in perceived self-defense. And they saw our buildup and assumed we planned to invade them, so they built up an even larger arsenal in self-defense, and so on in a vicious cycle.
That's why the only way to end that kind of conflict is to take a chance on peace, to try to break the cycle, as Kirk did. Pointing fingers of blame at the other side is just an excuse to keep fighting. It's not a solution.