Wow, there are some really strange ideas in this thread.
First, a lot of speculation. It looks like roadie might have been trying to cut off MTB dude. We didn't see the video before what was posted, maybe roadie provoked MTB before all this happened. Well, sure, maybe, possibly. There is nothing I see in the video that is definitive evidence of any of this.
Second, a lot of "that's the way the world is out there." Well, sure. And also, there are countries where HIV is endemic and a prevailing belief that you can cure it by having sex with a virgin, so massive numbers of HIV/AIDS+ men raping young girls to try to get rid of their disease. That's also the way the world is, sure, but it in no way justifies anything. There are lots of raging jerks in the world, this does not exuse being a raging jerk.
Others have critqued roadie's riding skills; while not accusing him of deliberate provocation, they say he's unaware of his surroundings or obviously not an experienced rider. Based on what I see in the video, this is possible. But since when is it morally right attack people in potentially lethal ways because they're inexperienced? If this is the standard, how is anyone ever going to become an experienced cyclist?
MTB guy attacked roadie in a way that could easily have resulted in serious injury or death, knocking him off his bike in traffic. That much we know for certain. MTB guy is certainly a grade-A jerk and well deserving of prosecution.
Everything everyone is saying in defense of MTB guy is highly speculative, and the video certainly offers no definitive proof of any of the accusations made against roadie. Sure, maybe it is possible that roadie is actually Hitler, transported in time from 1945, and MTB guy was valiantly trying to save the world from another holocaust. OK, maybe. But I'll need to see a lot more evidence than this video offers before I can find any justification for MTB's attack.