According to the source of the video on Reddit, this fight started after the drunken older man had been arguing with a group for about 10 minutes and was given many chances to leave.
The man wearing the ADCC hoodie (Abu Dhabi Combat Club, a prestigious submission grappling tournament) kicked off his footwear shortly before the video starts. Then:
Strikes -> clinch -> takedown -> mount -> armbar -> head stomps from armbar -> mount -> elbows.
Bystanders offered to call an ambulance for the older man following the fight, however he refused and kept saying that a bunch of “homeless guys” had attacked him.
As I’ve explained in a previous post, the standing vs ground debate is a red herring. Sometimes going to the ground is the optimal strategy, sometimes it’s not. In all cases, grappling skills are necessary whether you want to stay standing or not.
Saying “never go to the ground in a street fight” is idiotic, and at best a misguided oversimplification.
In this instance, the man who performed the armbar was not jumped by a group, was not stabbed, there was no glass and AIDS needles and lava on the ground.
It also starkly illustrates the effectiveness of join locks. This is not a submission, he did not wait for his victim to tap. He just destroyed that arm.
None of this is to say that his behaviour was justified, that’s up for you to decide. Where I live, it would be considered excessive and serious charges would follow. Make sure you understand local laws before using force.