First, an explanation of NPC behavior, primarily in First-Person Shooters: When an NPC sees an enemy NPC, it will immediately attack said enemy NPC with no more words than some Enemy Chatter. The same holds true if an NPC who is an enemy of the player sees the player. Even if the attacking NPC says "Put Down Your Gun and Step Away!" or "Stop or I Will Shoot!" they've already pulled out a gun and started firing. Except in a Cutscene, of course, in which they are perfectly capable of stopping and arresting the player. Can be easily justified if they arrest you when you run out of health.