If you saw somebody pointing a handgun in the face of an innocent by-stander and demanding money would you intervene?
That is a particular, and difficult situation. Approaching the gun wielding mugger openly is likely to get me shot before I can close enough to try for the gun. And even if I get there without being noticed if my technique isn't clean I will likely die. If, in that instant of seeing the problem I thought that the mugger would behave rationally and leave with the wallet without violence, then no, I wouldn't step in. If, on the other hand, I thought the guy was going to start shooting, then I might well try something. After all, if I'm close enough to intervene, I'm likely next on his list anyway.
Some kids stomping another kid - sure, I'll try to stop that.