What threshold would/could you pick?
For myself, well, threaten people I love enough and make me desperate enough and I will probably snap, even though I am the calmest and most balanced person I know.
A psychopath is someone who sees others as means not ends. Their existence is irrelevant except insofar as they serve his wants or needs, or prevents them being satisfied. A borderline psychopath, one of the people who was born that way inclined rather than straight born that way, and who was triggered by environment will probably be no more attached to any human being ever than you would be a to a pet.