"The scene is about how such a sane and serene person can be pushed to the limit and do something so seemingly irrational and passionate (threatening to thrust Ramon's hand into the garbage disposal). I think because it's so outrageous and out of character, Ramon is ultimately not as threatened as he might be. But most importantly, he's the type of cocky young guy who gets off on the passions he arouses. And so he stays."
That sounds like a very smart interpretation of what McNally probably intended for the scene. I can understand that you think it works on that level, but I still don't like it, because after such a moment, I don't think Ramon would want to stay in the house under any circumstances -- and, for that matter, it seems highly unlikely to me that Gregory would allow him to stay.