Well, I think that there are other reasons. Because first of all, he is now a king and even though he could be nice because Jack is his nephew and I think that it might be because he could be trying to trick them by being nice then he could lure them into a trap. Also I think  Xaafuun is harsh so I think that  she told Uncle Bud to trap them. Because in the story Xaafuun says,"  The navigator hesitated. "The entrance is still there, but I can't say how long it will last."Of course you can't!" She said as her temper flared." That proves that she was harsh at he moment. Also second of all, I don't think it is just of kindness because since they only have one reserved monster left, I think they might use Jack and Isadora as monsters since they probably need more they will lock them up in a higher security cell and make them fight other monsters and since Jack is very close to Uncle Bud maybe Xaafuun thought Jack will trust him and possibly Isadora so Uncle Bud can trap them for Commander Xaafuun.