I understand sitting shiva to be an expression of bereavement. If I (hypothetically) was the victim of constant abuse which lasted for years and I ultimately, in an act of self defense, had to kill my abuser (a close family member) would I still be expected to sit shiva and be constrained by other displays of aveilut? I wanted the person to be dead so I would think that mourning would be inappropriate -- I'd be celebrating having escaped the abusive situation and saving my own life.
Would a pro forma shiva/aveilut be required regardless of the logic, propriety or sincerity behind the situation? While I can understand that there might be a sense of loss to the community or some metaphysical sadness because this person has lost the chance to do tshuva and return to his people, I as the victim would not want to mourn the loss necessarily.
Does aveilut/shiva have to go along with a sadness or does it stand on its own as a requirement regardless? Would a cold blooded murderer of a family member have to sit shiva (I understand the dissonance between being willing to take a life and still feeling bound by other halachot, but there are Jewish criminals who keep kosher in prison, so...)[any help in tagging would be appreciated..I can't find a tag for self-defense]