Beelta asked him if what is happening to her is normal. It is a very good question. Unfortunately, she is asking the wrong person. He doesn't know. If he knew, he wouldn't come here looking for test subjects now, would he?
He should take offense at her question. A more volatile demon king would smite her for such insolence. Fortunately for her, he is a calm-headed demon king that relies mostly on logic, careful observation, and analysis to make decisions. He can see that she is going through a lot of pain which might have compromised her mental faculties. So he won't get mad at her.
He would be a bully to punish someone that is mentally compromised for asking a question of ignorance. That's his emotional reason for not killing her for her insolence. The logical reason is that killing her will rob him of the rare opportunity to study the effect of his unfinished good on a demon lord. So he continued to watch her as she groaned and spasmed.
