Say your out and about, and you happen to see an assult underway, where it appears to you the assilant is threatening someone's life.
Since the assault was underway I take it you mean the assault had started and you didn't see the beginning. Now how do you know who is the perp and who is the victim? Just because someone has the gun doesn't make them the perp. Suppose the one with the gun was defending himself from a strongarm robbery.