ToME: the Tales of Maj'Eyal

I entered the Assassin Lord's tunnel to save the merchant as a halfling rogue. While stealthed I Positioned myself adjacent to the merchant and assassin lord. I talked to the lord without talking to the merchant. I said I thought there might be treasure then I said [attack]. I immediately hit the assassin lord with dual strike to stun him. Then I popped Luck o' the Little Folk and struck with whirlwind killing the merchant in a single blow

The merchant probably shouldn't be a valid target, just like the yeek wayist.

Lots of skills kill the merchant you're trying to save. It's not just whirlwind. It's the same with escorts and Melinda, really. Cast pyrokinesis with Melinda next to you, she burns to death. Catch your escort in a Scattered shot, they're toast. It's part of the challenge.