Are we talking about reality or imaginary things?
If we're talking about fictional things, then I would make everyone love each other :x
It's harder to kill someone when you love him/her...
If we're talking about possible things, in that case I would just make everyone stop hating each other
Ok, for real this time: I would make the women equal to men in every respect; education, possibilities to make decisions etc.
Don't know if that's any more likely to happen than the two above, but in theory at least.