I guess I interpreted (or fanwanked) the bar scene differently than everyone else.
When Cas & Dean are talking to the bartender Dean is showing her his badge. And when they first sit down at the table Cas is showing Ronnie his badge so they are obviously supposed to be FBI agents. Dean doesn't even do the obligatory I'm Agent whoever and this is my partner Agent Whoever and we are investigated a case. He goes straight to "bad cop" and when he slams Ronnie's head into the table Cas looks back at the bartender nervously.
Later when Cas says Dean lost it I thought he meant that Dean completely forgot the part they were supposed to play. The bartender knew them as FBI agents and Dean was manhandling someone she had just pointed out to them. The head slam wasn't that bad in and of itself but Dean is usually much more collected when he is interviewing a potential witness/suspect. Especially in a room full of people.
YMMV.