Allow me to set the scene. My players have been clearing out an ancient temple infested with goblins and the servants of an evil necromancer. They have just defeated a group of mercenaries and a doppelganger, who is now begging for his life.
Doppelganger: Please! Let me go! I’ll leave and never come back!
Zarok (Human Fighter): Do we have your word on that?
Doppelganger: Of course!…
Zarok: So you won’t gather some of your friends and come back to fight us?
Doppelganger: Why on earth would I do that? You just killed all my friends… But what’s to say that you won’t come back and kill me? How do I know you won’t do that, hmm?
Ursa (A Half-Orc Cleric, who up to this point has said nothing): YOU DON’T!
*Rolls a 20 and splits the Doppelgangers head in two with his battleaxe*