I think it is also important to understand your role going into the situation. If you are a "professional photographer" on assignment, you will have a well defined mission with rules of engagement. If you are not given an assignment, or are not paid, you have a different role. Even still, we are all humans first, and we must all act within our own set of values. If our values are crystal clear to begin with, the decision may be hard, but what the right thing to to will be obvious, as our values do not change- just the situation.