Cutting the woman's head did not guarantee she would not revive again. But Kyran did it anyway to ensure her soul would not have a complete dummy body she could use. Besides, he felt that Layla's spell was connected to the dummy. He guessed as much because when she revived, the dummy body reattached its severed head and even healed prior injuries. He also guessed that as long as the dummy did not sustain irreparable damage, the soul connected to it would continue reviving.
Kyran could have destroyed the woman once and for all using the Void Sphere. But if he did that, he would miss a good opportunity to check how Layla's spell worked.
Moreover, Layla was aware of Kyran's magic. Since she had used the dummies, knowing that he might appear, she likely had prepared a countermeasure against his magic.
