All of your concerns ARE valid... no question about it. It's too bad we all have to add such considerations to out total thought process when considering whether or not to act.
Me... I'll roll the dice in a pure active shooter situation. However, should I see two strangers getting into it... I, as a civilian, perhaps would not be so quick to act for the very reasons you pointed out.
When we conduct our annual HR218 qualifications, the retired cops are CAUTIONED very strongly against injecting themselves into a situation between two distinct parties. You DO NOT know WHO has done WHAT to initiate the conflict... thus you could very well take action against a real victim w/o knowing it.
All good points in this thread to consider... in the end, the INDIVIDUAL has to retain the final say-so as to what he or she is willing to do w/o pressure from anyone or anything.