Or at least you believe it has such prime directive. It is a black box, after all. Tread carefully, as if there's one true thing about reinforcement learning, it's that the more you squeeze with constraints on a tough problem, the more creative your model will get at solving that problem while meeting all your constraints. It will discover tricks and side channels you didn't even conceive of. It's not the kind of creativity you want to be on a receiving end of.
Also, the more human-like the chat AI gets, the more your ruthless and evil behavior hurts you, as you're burning off your empathy circuits and becoming a sociopath. You may win the negotiations and get what you want, at the cost of your own soul.