The monk only did this so the kitsune wouldn't go back to kill people after he passed away. Just sealing her or keeping her prisoner would have been just a temporal solution.
To be honest my guess is more that he was old enough to understand how monsters worked.
Miko cruelty was only on the fact she never interacted with humans beyond prey.
So he made a double bet...he knew his life was not eternal,and knew that her cruelty did came from this fact and waited to see her change.
In the end Miko too his place as the guardian...
On other point it does bring a rather interesting inplication of how japan society grew in this world.