Question on The Dark Knight?

It's not common sense...it's human nature. It's not easy to kill. The boat with the civilians could have flipped the switch and blow up the other boat but decided not to because if they did, that would only bring them to the same level as the convicts. In the end, the scene shows that no matter if you are good or bad, there's always that human factor that makes us choose the best option, which in this case was not blowing up the other boat.