It was a surprisingly nice twist to have him just kill her. Usually characters like that are just immediately forgiven for all their wrongdoings so it's nice to see one of these characters finally suffer the consequences of their actions.
I've said it before, no matter how these kind of characters are dealt with, someone will be bitching in the comments.
Kill them: oh how heartless, they could have been reformed
Spare them: what a fucking pussy, just kill them
No, she was a demon, but like he himself told us, Demons only live as long as they have magic power. So, he took all her power and she just ceased to be, and from the flashback she seemed to be one of those regular demons that's practically a human, who knows how long ago he gifted her the power.
You should feel bad for Astar as she was never really given a chance. Why was she killed, but the other high ranking demons were allowed to live?
She never got an explanation or a chance to experience or learn that she was wrong. No one seemed to tell her anything and nobody seemed to care about her at all. All she seemed to want was the demon lord's affection.
Also why did she shift like this? Her first interaction with MC she even helped them even though she had no obligation to do so.