When to engage a suspect? And levels of response.
Person is unarmed and punching people, including NPCs, deploy Taser or Airsoft and cuff suspect.
Person has a firearm out, not pointing it at anyone, this is an open carry state. If the person aims it at a person, including an NPC, you have the authority to engage this person.... meaning you may take your firearm out and fire at them and eliminate the threat.
If you're on foot and someone is going to ram you with a car, you have the right to defend yourself by firing at the inbound vehicle.
Person has a knife, have your pistol out. If they run towards you, you may open fire to eliminate the threat.
Person has an automatic rifle out, be sure to have yours out too, and follow the rules of engagement above.
If the suspect gives up (They put their hands up), you must cease all lethal means and detain the suspect while being as safe as possible.
If a suspect puts their gun away after shooting, and are actively running away, you cannot shoot a suspect in the back.