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.
What he gave her isn't the Chaos spark, it is "supposed" to be an upgrade on the Chaos spark. The key word is "supposed". It is in fact, not an upgrade. At least not yet it is not. That's what Beelta's current suffering is for. It is to contribute to the completion of this "upgrade."
He has made some progress in modifying some of his sin abilities. This chaos spark upgrade is just like this avatar upgrade that he created with the aid of the black orb but he has not tested it yet on a demon Lord being. He can't tell her if her reaction is normal or not because he has never done it before. This is an experiment for him in every way you look at it.
Creating an upgrade for something that can empower demons is a tricky business. He had to experiment on mid-rank demons for him to modify the Chaos spark. The Chaos spark has reached its fullest potential now so he needs something more powerful if it is to remain relevant to him.
As a demon king, he can create demon dukes without the Chaos spark. Every demon king can. But upgrading a demon king to greater heights of power is much more difficult. He needs willing candidates that he can sacrifice for the cause. So he needs test subjects.
He is not willing to use his subordinates because they have more value than to be test subjects. He didn't groom them by giving them resources and removing the weak ones just to use them as test subjects. This means he has to find other test subjects who are either willing or who will need to be coerced gently to become dedicated to the cause. Beeta is not the first test subject but she will be pleased to know that she is the most powerful one up to date.
She underwent rapid physical changes as he watched her. Her body began to enlarge and warp. She increased in size but her maws enlarged faster than her body. They became enormous attachments to her body so it seems that she is more maw than a body.
Strange growths began to appear too. Another arm or tongue, or the lone tooth appeared on the small part that is her body further increasing the burden on her. The black sphere is supposed to make a demon duke/demon lord more powerful. It is not working apparently. Which to be honest, is not much of a surprise. He at least is not surprised by the outcome and that's what matters. Beelta, being the stick in the mud that she is, did not think so.
She shrieked, "You tricked me. You wanted to kill me."
The specter answered honestly and with some mild indignation. "Now why would I do that? I can easily kill you if I want. I just wanted someone to test on."
He honestly has no ulterior motive when he offered her the sphere. His intentions are as pure as they come. He wanted a powerful guinea pig to experiment on and that's all he wanted from her. The fact that she believed otherwise is just her fault.
He has been testing the black sphere on some random demon nobles that he caught in the plane. Not every demon noble followed the demon lord on her dangerous wild goose chase. He found those demon nobles and convinced them to contribute to the cause. They agreed but they always explode. They die instead of evolving.
It was a very disappointing result. He tried everything he could do but the outcome doesn't change even if they swear fealty to him. So he thought the black sphere might be too strong for them. He decided to find a much stronger test subject and Beelta volunteered. She has no one to blame but herself.
She began to rant. "I won't die without repercussions. You will pay for it if I die. I am protected. I can't die..."
He ignored her rantings but that didn't discourage her. First, she threatened and warned him. Then she began to plead and make various promises. Then that devolved into incomprehensible sounds of pain accompanied by abominable changes to her body. He noted every change as he observed her intently. He did this all the while being calm and level-headed.
Her body grew into a huge, round grotesque monster with multiple sets of eyes, maws, tongues, arms, and legs. She was crying in pain throughout the whole process. Then she became silent after 10 minutes. The pain and the chaotic changes finally broke her mind. She couldn't feel pain anymore. In a way, she has died. Beelta is no more but her body continued with the gruesome changes. It took more than 30 minutes before her body had had enough.
She didn't die peacefully. Her body began to glow a dark light. It was ominous and it is also what the other demon nobles behave before they explode.