I'll agree that within this context "monster" has meaning for Sam beyond what Dean probably intended. On the other hand, it's always been "WE're freaks" and now it's "YOU're a monster", so Dean is either changing his tune significantly or he means something different by "monster". Even if he didn't know the significance it had taken on for Sam, he didn't intend it as a complement. And there was no indication that they weren't both equally aware of the significance "if you leave, don't come back."

As for wanting a fight, there were points where they both reined in, trying to avoid one. Also, the show very pointedly has the conversation with Bobby advising Dean that the confrontation with Sam needs to be "about getting him back, not pushing him away."