Remorse is an emotion experienced by one who feels they have committed an action contrary to their moral code. It is characterised by feelings of regret, self-hatred and a desire to make the wrong thing right.
The idea of remorse is used in restorative justice.
One incapable of feeling remorse is often labelled a sociopath (US) or psychopath (UK) - formerly a DSM III condition. Some researchers have lately suggested that this lack is more characteristic of the INTJ personality, a highly rational temperament that relies very little on emotion. In general a person needs to be unable to feel fear, as well as remorse, in order to develop psychopathic traits.
"Buyer's remorse" is the concept of regretting a purchase after the fact.
Remorse 
