When you think about it carefully, you can see clearly that Yamu should be a highly intelligent creature. Naturally, in such a situation, there will be many preparatory plans. It has long been thought that they will violate what they said before, and then kill Yamu. After all, this fellow is constantly providing the ability to restore the witch king. In such a situation, Yamu will be killed. They couldn't find any reason to let this creature go.

They just forgot one thing.

Yamu is a very smart creature. This kind of thing was probably calculated a long time ago. So before they killed Yamu so rashly, it's very likely...