And it's just that simple right?
Someone gave me a really good perspective on whether to make that choice to get involved or not. Imagine for a moment one of the people we're talking about saving developed a curable form of cancer but they had no money and just needed $20k to live. Would you write them a check for their medications? If no, then why are you so eager to run into that situation with a gun?
While not quite apples to apples, I think it's a great analogy for deciding whether or not to get involved. In all but the best of scenarios you're going to need to hire legal council and your name may get drug through the mud (ie: Zimmerman) before you finally get clear of all the legal hurdles. By the time you factor in attorney fees, lost wages, all the possible negative attention you'll receive and stress $20k might not even come close to covering it.
I have had two instances in my life of intervening on behalf of an unknown woman getting smacked around only to have the crazy bitch come swinging at me when things went sideways. How thankful do you think the masses will be for your actions? Remember, these are the same people that suspend teenagers who step in to defend others being bullied for fighting.
I'm not saying I wouldn't take action in any scenario, but considering all the problems (LEO shooting you, legal troubles, thankless masses, etc.) I'd take a moment to really think about it before taking action.