>How should we decide.
Since this is very personal, very much based on the background of a person (religion, country, culture), I don't think there is a general answer to it.
>If we want to find an ideal way (Theory) to decide .. ..
If there was a, mathematical based, theory to make decisions in real world, and if we would understand it, then we would always make the right decision. History shows in a lot of cases we, as well as individuals as groups, don't.
