Don't matter, it was in defense of life period. Standard procedure in the bush is if you find a bear out in the winter, kill it ASAP. For a bear to be out in the winter means it is either sick, starving, injured, or all three. Anyway it is going to be looking for a fast meal. We are not at the top of the food chain, we are just another pray animal to them. Therefore for the protection of the village they have had the policy for centuries, of hunting down and killing the winter bears quickly, before it comes back to the village and picks up a kid or two. That is just what happened here. Only difference is that after bear was found and killed they realized it was not a Grizzly, but a Polar Bear. Doubt most of the people in that village had ever seen a Polar Bear before. They had no reason to believe it was a Polar Bear. If it had been seen the residents would have just thought it was an Albino Grizzly, or it was just covered with frost.
What was done was justified and prudent. As for the AR-15, any gun beats no gun. He used what he had and it worked.