But you didn't. Video recording of the event changed your status from witness to participant. You didn't even report the attack on yourself.
But you did pull it out and record it and I am assuming from the "Canadian fella in the comments" that you posted it on social media.
A self deputized photo journalist maybe?
Cell phone video is part of our culture now. Here in ST&T we often use video to learn from other's experiences. But as you found out, and fortunately it wasn't a really hard lesson, many people are going to react poorly if they see you videoing their actions. Especially if they are engaged in criminal activity. You have to weigh if the FaceBook "likes" or the YouTube, Twitter or Instagram hits are worth serious injury or worse before you pull your phone out and start recording.
The proliferation of cell phone video is changing the dynamics of many things in our society. Light has been used as a force multiplier in law enforcement for decades. Yet now it's a cause in an ACLU lawsuit against the police in St Louis because it was used during the riots in the last couple weeks to help push the crowds back. The ACLU says that it's being used to thwart video recording. I was using light on traffic stops back in the days where everyone carrying video recording capabilities in their pocket was science fiction. We've seen a return to criminals wearing masks in an attempt to be anonymous on video.
If you are going to video record someone's conflict you should do it unobtrusively or from a position where you can't be reached, because as you found out, a lot of people don't want a video of their bad conduct out there.