I would want them to help me out but not fight the battle for me, we would stand up to the aggressor together.
Would i help out someone else? i would do my absolute best but if there was weapons involved then i might have to think about it first, im sorry but thats just the way it is, i would not just walk away though i would call the police. It all depends on the weapon as well, if it was a gun then no i would go near, if it was something else like a knife then i would consider the options as quick as i could.
Example of this is a few years ago near to where i live, in a totally unprovoked attack a woman was killed with an axe outside the front of a restaurant, in broad daylight with dozens of witnesses but nobody helped because they feared for themselves.