Russian gamers in general rage a lot. If you ever watch their scrims on stream, diamondez is Constantly raging at gosu pepper (If you understand russian that is). They're not joking around, they're just assholes :S
Yeah, that's pretty unprofessional, especially in a community where the pros are ~generally~ well mannered and well behaved with one another, often showing good humor about the games regardless of the outcomes. This definitely disappoints me to see from such an up and coming professional team. Perhaps they should work on their etiquette.
However, and though this is no excuse, they ARE in an environment where tensions are high. Mistakes on one person's behalf could decimate your chances of moving foreward in the ladder and no one wants to be That Guy. Diamondez probably took it as an affront to his skill and the fear of becoming/being so offended that someone could associate him with That Guy caused him to over-react like a five year old ;P
STILL unprofessional however. Even if it WERE an attack towards Diamondez, that's no way of handling a situation ;P
Brilliant players, just have to work on the "professional" aspect of their game ;P Basically, M5, Y U NO PROFESSIONALE?