Saw something going around on FB a while back. Had a guy in black ski mask, black clothes, standing in what was a school hallway, holding an AK type rifle and his back was to you. It asked you what you would do first.
If your answer wasn't: "Shoot the bastard", you were wrong. You aren't law enforcement. This is isn't the old west. You don't have to tell him to "Drop it". He doesn't have to be facing you*. He's a bad guy and needs to put down RFN !!! You don;t have to worry about being "Honorable" or making it a "Fair Fight"*. Kill the bad guy. Don't speak, don't flinch, don't hesitate. Kill the bad guy.
*- Unless his name is Simon Tam.
*- If you are in a fair fight, your tactics suck. Also see the great Philosopher Jayne Cobb for further thoughts on a "Fair Fight".