1. If you're religious, then you can point to an ultimate evil (or some cosmic principle) to handle 100% of something. Otherwise, divide and distribute proportionately.
2. In most events that are bad, the amount of responsibility is arguably greater than 100%. Yes, you can renormalize that, but it's still true that each side has a larger number of options (most of the time) to avoid a problem or conflict than we like to admit at the time. Many options are utterly unpalatable. But in hindsight, there are quite a few solutions that are far better than the mess of a solution opted for.
WWI was avoidable. The Civil War was avoidable. WWII was avoidable. The Russian Civil War was avoidable--as was the Revolution and the gambling that helped produce not only WWII but 21 million or so dead at the hands of socialist Stalin and more at the hands of Lenin. (Call them "communist," but that's like saying pre-Millennialist Xians think that they're already in the Kingdom and Jesus is ruling, now, over the Earth. Communists believed in a future, as do the PM Xians. The Union of Soviet Socialist Republics was aptly named. As Stalin said, they were building--right up to his death, "building"--socialism in one country. The rest is just S.)
Nobody thought was possible or likely so the solutions were demoted to unpalatable. Take the Civil War. Now, show those driving towards war the outcome of the war. Include the Northerners in that show--divided, like male and female in various religious services. And ask if the death and bloodshed is worth it--and put it to a vote. See if the line between good/evil doesn't do a bit of a shift.
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