Here is a defense of a stranger shooting in Missouri that we discussed in Tactics and Training:
http://thefiringline.com/forums/showthread.php?t=454222
As that account shows, it is a lot easier to get into trouble with third party shootings than it is to get out of it. Another good example is some of the Force-on-Force training from the Spike TV series "Conceal and Carry School" (available online at:
http://www.spike.com/full-episodes/t...eason-4-ep-113). It provides some reality-based scenarios showing how quickly the person you are saving can become your attacker.
Given the degree of uncertainty involved in those situations, I would probably opt to be a good witness and call police unless I had a very, very clear picture of what was going on. And even if I had a clear picture, I don't know that I would be all that excited to risk my life, my freedom, and my financial well-being to save someone else who didn't value their own life enough to get a CHL and use it.