at issue is felt the hidden sadomasochism :))))
as you know each of us has a small fraction of sadism and masochism, and if she crosses the threshold of what is permitted then it is a pathology - direct road to the asylum or to jail. thank God that man is still a rational being in us from time immemorial ancestors put such cultural notions as morality, religion, culture, honor and respect to similar (although much has worn off in the genes - there is a genocide, and more).
man within himself is a prisoner of his emotions, along with this is bound the moral and ethical norms, and the beast inside him is constantly trying to escape and to disregard those rules and regulations imposed on it by society. and if people step over the torque (moment of weakness) then it ceases to be a reasonable person and becomes merely a wild beast.
summarizing all the above, and switching the question... just not a healthy person may offer to commit murder dari love. even though we consider them reasonable no - kill only to survive... and only a human - type intelligent creature kills for the sake of pleasure...