They played it safe. As long as those two thugs were not threatening them, they had no quarrel. It's a good question. I think we all wonder what would allow this sort of thing to happen and what can be done about it. Inherent in our behavior, from how we build our houses to how we order our lives together is selfishness.
Human nature is basically selfish.
I'd like a member of the audience to come up here for a moment. I pinch the member on the palm of her hand. It is physically impossible to feel someone else's pain. A head wound. Like a lot of people, I used to believe that human nature was basically good until I was mugged by reality.