A question of moral obligation. I go back and forth on this a lot. If I saw a man grab a woman into a dark alley at night, and I'm the only person around to help, it would be a pretty crappy thing to do to let it happen without any level of intervention. However, people do it all the time, especially in places like NYC. Perhaps a more obvious example would be a baby that crawls into a pool. If the baby crawls into the water, where it will inevitably drown, the person who sees that baby would have a moral obligation to intervene.
However, I don't believe that a government has the obligation to intervene whenever a malevolent dictator abuses their own people. Nor do I think that if a stranger is getting his ass kicked by 4 dudes that I would need to jump in and try to stop it. I'm not going to spend the rest of my life breathing through a tube or getting shot because some asshole spouted his mouth off to a guy and his friends, resulting in the whooping he might have deserved.
I guess it ultimately comes down to cost for the intervener. If they can stop an evil without undue personal costs, they should probably do so. Right?