You've got a point. Jaheira hasn't been shy waving around her tadpole in a jar. Surely the harpers would know about tadpoling and I presume they briefed the Flaming Fist. Who have eyes and brains anyway.
Maybe it's just war-think. Calling them traitors helps them kill first, question later. You don't want disadvantage to your initiative.
Even so, it'll be better to take him in alive than dead, so you can torture him (Hold Person + Heat Metal) for answers, also good to have a healer nearby to keep NPCs alive long enough, it's what I would do in D&D.
