Mindfulness actually is the line. When you are unmindful, such as when you are in a rage, you are indeed not different from a madman. It is only a matter of degree. When you have zero mindfulness over an extended period, then you can safely conclude that you are crazy. So, if you don't want to go mad then hang on to your dear mindfulness, for on the other side, that is, at the bottom, is living hell.
Sanity is indeed a thin line.
It really does not need much, so to speak, to break a person. Lock him up without contact for a few days or weeks and there you are, someone fit for the mental ward. If you are a meditator, you'll have a fighting chance and maybe even end up saner than anyone else. But they may still call you mad because you are different. I remember how some people considered a friend mad because he was not his usual excitable and irritable self after a retreat.