Wow, how did that work? Was the NPC statted already or were they a non-combatant?
The NPC was a mid-level tiefling (encountered in the Well of Demons in H2 Thunderspire Labyrinth). In the module, the two tieflings are statted up.
As I ran the encounter (described in more detail
here) the tieflings were unlikely to come onto the scene until a whole bevy of gnolls (plus there hyenas and demons) had been dealt with. And indeed that was how it happened.
When the PCs started negotiating with the tieflings, it turned out that the gnolls had fed them rot grubs and one of them collapsed an reanimated as a rot grub zombie, which the PCs dealt with fairly handily.
The other tiefling then followed them around the rest of the abandoned temple of Baphomet, with a status ambiguous between that of rescuee and captive. He tried to persuade the PCs to do a deal with his diabolical patron ("The Master Trapped in Ice") but while one PC was quite keen on this, another - the wizard - was vehemently opposed.
The climax of the scenario was stopping a gnoll warlock from rededicating the temple to Yeenoghu. This involved stopping a ritual - which, naturally enough, led to the unleashed magical energies bringing the temple collapsing down!
As the PCs fled for the entrance, the player of the wizard stated that he wanted to execute the tiefling NPC with a Magic Missile. There seemed, to me as GM, to be no good reason for insisting on playing this out as a full-fledged combat, when the outcome (8th or 9th level PC wizard vs 6th or 7th level tiefling NPC from the MM) was not in doubt. On the other hand, the other players were likely to be disapproving of this, and it seemed fair that they at least get a chance to intervene. So the player rolled an Arcana check to go along with his Magic Missile. He made a Hard DC, which I decided was enough to "minionize" the NPC, and hence have him die automatically from the Magic Missile. The other players (and PCs) were then unable to do anything but express their shock at the wizard's ruthless behaviour.