Quite simply, I wouldn't.
As much as I detest seeing certain things happening (murder, hatred, greed, etc), I've always believed that everyone is the sum of all our experiences.
If I hadn't been beaten on so much at school, I'd have never learned to be nicer to people (surprise surprise, I was a little shit when I was a kid) and never learned to stick up for myself. If I hadn't been rejected in love so often, I'd have never opened out and come out of the closet... nor met my wonderful boyfriend.
I've always followed the saying that what doesn't kill you makes you stronger. The difference is that it's not just the stuff that could kill you that do that, it's everything. I've become a much stronger person throughout my life and if anything had changed, chances are that I wouldn't have. I know this isn't the case with everyone. Some people (eg victims of abuse, etc) tend to lose their confidence and strength because they're hurt or whatever's happened.
The same can be said for humanity. Everything that's happened has made us stronger as a species. Anger and greed bred dominance, which as time went by slowly grew into proper states of leadership. For a more recent even, look at any weapon that's tested by being used on someone/people... the most famous example being Hiroshima. Whenever a new type of weapon is misused, humanity realises its error and despite there being some fools who still try to misuse the weapon later on, as a whole, humanity knows it should keep the weapon locked away and NOT use them. I mean c'mon... how many nukes have you actually heard of being used outside of testing grounds since the Fat Man and Little Boy were dropped on Hiroshima?
Everyone and all of humanity is grown with every little experience. To change any part of it would dramatically alter how we've grown and make us weaker.
