Alright so this should work anywhere where you can duel with guards around (I tried it out in Thrallmar, just a note).
So I'm fresh out of Scholomance and just got the Barov Peasent Caller and I head to Outland (Summons 3 Minions to fight for you..the trinket is pretty bad but it makes for some confusion if you use it in a duel). I dueled a Death Knight and popped cooldowns (which included the trinket) and the Peasents starting attacking him. Since they have very low HP he was able to kill them very easily but after he killed them about 3-4 Thrallmar guards started to kill him..pretty funny stuff.
My guess is that the Barov Peasants are counted as actual friendly NPC's and when you kill them the guards trigger a defense mechanism. Anyway, I've replicated this twice in Thrallmar, haven't tested it anywhere else.