One feature of commonsense moral thought is the doing/allowing distinction. It's generally believed that it's worse to do some evil yourself than to allow another person to do the same evil, even if you have the power to stop them from doing it. I think this distinction plays a big role in shaping our social and political behavior -- for example, why we're unwilling to inflict corporal punishment on criminals, but willing to imprison them in situations where other inmates will abuse them. One of the most unfortunate ways this distinction operates in the world is by inhibiting action that would make faraway suffering people better off. Since you're not doing any actions that make the people in Sudan suffer, you're not obligated to act so as to prevent their suffering. Sure, it'd be great if you did something, but no obligation binds you.
One of my favorite things about the kinds of heroes we find in fantasy is that this distinction has less of an impact on their behavior. Spider-Man holds that great power comes with great responsibility, and he's one of the best recent examples. He has the power to prevent others from doing evil, and he feels obligated to use that power to prevent as much evil as he can. When he fails to do so, he's wracked by guilt.
