"Do it." (x)
I understand how Sam feels at this moment…
Theory time, with help from cutsiecastiel
So, the blade can’t actually hurt Dean, as he is a Knight of Hell, so, what if the reason Dean is pushing for Sam to cut him, is so that demon blood gets on the blade?
What if, Sam relapsed into drinking demon blood, which is why Dean called him a monster, and the reason Dean is attacking him, is so Sam will become violent in the hopes of turning Sam into a demon?
What if Dean wants to rule Hell with Sam?