Jack didn't entirely reject using the Immortal Flame Sword, but when he wanted to use it, there was absolutely no opportunity.
At first, he indeed hadn't planned on using it, but he had completely lost his ability to resist too quickly. At that moment, he knew that even if he drew his weapon, there was a high chance it would be taken from him.
If the sword hadn't been taken away, Spamati would surely have noticed something unusual about him. He believed that, at that moment, the chances of him being killed would be very high.
Though Jack understood that the laws of this world weren't much different from the place he used to live, he wasn't a fool. Laws are made for profit, and once the profit becomes large enough, the laws inevitably change.
At this point, the system suddenly spoke: