I like Manny, but how he reacted was completely ridiculous. It seemed like he did it just to get attention on himself. If you notice when Nate is going off Manny doesn't even care what is written on the wall.
Then of all of a sudden he starts going off, about how he's going to "pack his bags" and go home.
Emerson is a big toolbag for writing that crap on the wall, but there's no need to get THAT pissed off.
You're just a cook...