I would want to watch, and if need be break it up. People do stupid things. If I see someone beating on a guy like that I'm in there in a heartbeat, because I know that the guy might die. And that's tragic, tragic for the family of the deceased, tragic for the ones that go to jail, etc. When I break up fights I don't go in there with the mentality that I'm fighting, I just go in there like; Hey now! He's had enough guys, don't do anything stupid now!" 90% of the time people stop. Once I protected one asshole with my body against 5 guys beating on him. He deserved it, but he already had hi ass beating, and at this point they were just using his body as a punching bag to went their own fear and aggression. It's pack mentality as well. None of them attacked me.
If I see two chumps fighting on the street tho, I stop a few meters away and just observe. Usually they end up rolling around on the ground gassing, after 20 seconds, then they go their own separate ways.
I have a lot of experience breaking up fights as I've been bouncing for 12 years now. One of the most important safety measures against being beaten to death is actually witnesses. In my book, these bystanders did a good job of caring.