Get Rid of Guilt

Guilt is a deeply human emotion that everyone has experienced (at least) once in their lives. They act as a warning signal to us that we are about to commit a wrong, that we are violating our moral code and creating an imbalance between us and one or more other people. In hindsight, help us restore the violated balance, make amends, and learn from our mistakes.
But why do some people tend to feel inappropriately strong guilt for saying "no" just once, while others can literally walk over dead bodies without batting an eye? How do medicine and psychology explain these differences? Where is the line between normal and pathological feelings of guilt? And most importantly, what can I do if I feel guilty about something I've done?