The Peace Prize has a very poorly defined mission.

Some third world activist who leads a social movement earns a lot of warm fuzzies, but probably doesn't affect the world very much (let's say Maathai).

Someone who pursues a naturally partisan and controversial goal peacefully probably produces a lot of conflict, but less conflict than they would have if they were violent about it. (Gandhi)

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 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).

And some random person in the right place at the right time may have a very large effect in terms of sheer scale, but be questionable in terms of genuine virtue (Petrov).

If about 60% of people, in Petrov's situation, would have done what he did, is it better to give the prize to him, or to some activist who has spent her whole life tirelessly struggling for freedom despite adversity?

I'm not a big fan of the Nobel committee's decisions, but given the pressures they face and the confusion of their task I don't think they've done a ridiculously inadequate job.