Perhaps the idea should be more along the lines of "Everyone acts in a manner that they believe is in line with their best interests". Break down why you do the things you do, and you'll arrive at the same conclusion.
In other words, regardless of how we interpret an action using our own experiences as reference, all individuals are doing exactly the same thing. The important thing to note is that everyone has a different belief system and therefore the view of 'acceptable conduct' varies wildly from person to person.
People do not usually act in a manner that is not consistent with their beliefs, and when they do they frequently experience guilt.
In a conflict resolution sense, the important thing to realize is that people do what they do because they believe that their action will bring them an outcome that is deemed highly desirable. Not everybody wants the same thing, and simply because lots of people think in a contrary fashion to that person does not make either "right" or "wrong".
There are laws, of course, but what society deems acceptable behaviour is not the issue. Understanding the motivation for an action is simply a matter of understanding what an individual believes will bring them a desirable result.