I don't see that "fight or flight" are the only two options, here. I don't think leaving someone who's willfully destroying his life (and dragging you down in the process) is "fearful" or "cowardly." Nor is it a declaration of war. You can simply open your hands and release him to do what he wants to do. Put him in the hands of his own higher power.
The "courage" part comes in, I think, to do what YOU were put on this earth to do, and to be. Your life won't go on forever, either. None of us knows how long we have. Do you want to come to the end of YOUR life feeling like all you were doing was treading water and trying not to get sucked under? You are obviously a bright, compassionate, articulate woman. There are things you could be doing with your life other than standing by watching his life circle the drain.