In the second season, Hannah, a female demon, made a contract with torrance, who has Char's body. No one can get Char's soul, so when the contract was signed, Char became a demon, and Sebastian killed Claude, so Char became a demon.

Although it was a breach of contract for the deacon to kill his master, Sebastian finally laid his hand on Charles' stomach. Besides, if Charles becomes a demon, Sebastian won't have a delicious lunch. You said he couldn't do it, but it was too late. When Charles opened his eyes, he became a demon.

Charles became a demon, but the devil can't be killed. Sebastian didn't want to kill Charles. He doesn't have any ideas. He did and carried it out, but he was a little late.

Moreover, Sebastian signed a contract with Charles because he wanted to eat his delicious soul after helping Charles fulfill his desire for revenge, and the soul of Charles who became a demon was not delicious and could not be killed. What do you want Sebastian to do?

As for why Sebastian is still a deacon of Charles, because the contract between them still exists. As long as the contract exists, Sebastian will always be a deacon of Charles and Sebastian will always be a deacon of Charles, so after Charles becomes a demon, Sebastian will still be a deacon of Charles.

If you still want to ask why, because of love. Sebastian has been a demon for hundreds of years and has no feelings for Charles. However, before Charles became a demon, Charles was alone. How can Charles have no feelings for Sebastian? The contract they signed will be valid forever. Do you think Charles will be willing to let Sebastian leave like this? What's more, Charles has no parents and no one to rely on.