A Principle We Should Keep

In Cormac McCarthy's The Road, we see that a key principle that the man and the boy are keen on keeping is never resorting to cannibalism no matter how desperate their situation gets. We also see that this principle isn't followed by some that the man and the boy encountered in The Road. The man and the boy is keeping this principle because cannibalism is the last straw that strips away a person's humanity. In The Road's post-apocalyptic world where hope is gone and death always surrounds them, the boy and the man aren't going to allow themselves to stoop to the level cannibals, in order to keep their humanity intact.
I think that in our world, a principle that we should keep is to not harm any other person, unless the purpose is for self-defense. I believe that no matter how far into the future we get, people shouldn't harm others solely for the purpose of harming them. There are also instances where the reason for hurting another person is for revenge because that person had hurt them in the same way, but I don't think that revenge solves anything. Harming another person will only cause more trouble and won't relieve the pain that they had felt. When we hear about people intentionally hurting others, I think that the two most common reasons for their behavior is either for self-defense or because they wanted to instigate revenge on someone else. Revenge only allows people to take their anger out on another and when they do, they aren't making the situation better in any way. Revenge never fixes the situation, and often only makes things worse. I see self-defense as being the only acceptable excuse, if any, for intentionally harming other people.