Agreed. But I realize that my point is perhaps in part about something beyond sanity: conscience. It is about not just knowing the difference between sane and un-sane, but having the decency to ask the question in the first place!
I've never doubted my ability to know sane from un-sane, and I think that's why I've never had issues with other people being sadists when I had doubted my own intentions. When it came t others I've always been fairly confident I could know the difference between sane and insane.
What I see as the risk is stopping to ask ourselves the question if what we're doing is actually a good thing or having the honesty to answer ourselves truthfully. It doesn't have to do with whether this or that level of extremity is abuse. For me abuse doesn't have to do with whether it's sunday spankings or total power exchange. For me abuse is whether you stop having your partners welfare as the motivation for what you're doing.