The leader of a very large country, like the United States or the Soviet Union, can have a greater positive influence on the world just by being a fraction of a percent nicer than the average person, than the leader of a small country, or a private individual, can have by being an amazing saint (Obama).

On the other hand Robin Hanson will vote against Obama even in a simple election because he, in Hanson's judgement, started a war unjustifiably.

Some dictator or lunatic who mellows out and murders less than usual probably has a very large beneficial effect on the world, compared to his usual murder rate (Arafat and the Israelis).

The "slightly less of a warmonger than you used to be" prize? I don't think the mission is quite that poorly defined! That said, he shared that year's prize with some Israeli folks so it was more a bipartisan honor for a specific act than in honor of the person. That is perhaps justifiable.