Although Kelin provided a lot of information that seemed valuable to Perfikot, in the end, Perfikot decided to simply kill him rather than let him go.
It wasn't that Perfikot didn't want to keep her promise to release him, it's just that she had no way to control this captive.
Perfikot had only captured him in the dream, and once released, he would immediately awaken in his real-world body. Perfikot had no idea where he was, making it practically impossible to capture him again.
Unless Kelin was a fool, and upon being released would stay in place to be captured once more by Perfikot.
As for completely destroying his spirit, or planting some suggestion in his consciousness during the dream, turning him into a puppet under her control...
Perfikot was an Alchemist, not a master of mind control. Even though she could use 'omniscience' to find a method for achieving her goal, she couldn't accomplish these things within the dream.
So it was more straightforward to simply kill him.
