I've seen all sides on this one...this one neighbourhood I lived in about 10 years ago was pretty rough (Main and Broadway, Vancouver) and you had to be on the ball all the time.
It worked like this - if you saw a domestic on the street you stayed about 50 feet away and made it obvious you were watching to see it didn't get too out of hand. I was doing this for quite a while before I noticed there would always be a couple of other people doing the same thing. Nobody ever acknowledged what was up, but all parties understood - your business is your business, but you go too far and we'll not just be on your ass, we'll be talking to cops later.
This was a very anti-cop neighbourhood, so cops weren't called for petty stuff, but there were things you just did not do, and smacking your girlfriend on the street was one of them.
Guys fighting each other were let to fight it out unless it was a grossly unfair match and/or weapons were being used. Then you'd get bystanders yelling and threatening to kick the shit out of both combatants unless they knocked it off. Cops were rarely called.
What no-one did was get in there between scrappers unless it looked like somebody was actually going to die. My friend learned that the hard way. His next-door neighbour was beating the crap out of his girlfriend and answered my friend's knocking at his door with a baseball bat, and my buddy is permanently blind in one eye as a result.
There is no right answer to this one. It depends on where you are, who's fighting, why they're fighting, how they're fighting, if you're from the same neighbourhood or a total stranger, if your social status is similar to the combatants or very different, if either of the fighters knows you, and how many other people are around, and where you think their loyalties might lie should you jump in.
Nothing like trying to be a hero and suddenly having half the neighbourhood kicking your ass because they know the backstory and you don't.
My advice? In most situations, standing far enough back so you can book it if you have to, yell that you have ALREADY called the cops, and make sure they can see you on your cell. At that point, call the cops.
If it looks like the cops aren't going to get there in time, and you think somebody is actually being killed, only jump in there if you have a shielding weapon - something you can grab, like a realty sign off a lawn, a sandwich board from in front of a store, anything, and sucker the guy who's doing damage. You'll probably only get one shot, so make it a good one. You want to distract him long enough for the cops to arrive or the victim to get away.
Then get the hell out of there.