This is one of those situations so often discussed here ... you come on an incident in the middle; HOW do you decide who the good guys are, and who are the baddies? Who do you shoot first? How do you explain why you got involved with a weapon to your jury?
Get through or turn around, get to safety and call the cops!! They get paid to sort out messes like the one you described.
"The welfare of humanity is always the alibi of tyrants." Albert Camus