@Caglio the reason the magic is so powerful is probably because it carries such a risk to the user. So I imagine once it is activated they have to accept the result. It's a pretty common concept used in fantasy stories.
You'll also note that Frieren the Gravedigger waited to reveal her hand until she was sure the magic had "stuck".
And also that she didn't keep the demon as a slave to kill other demons with, since the demon might figure out ways to work around the magic. Just a quick order to suicide - even that in the simple words of "kill yourself".
So played it very safe, just like she has done for a thousand years..
Best humiliation for a demon is suicide. Though sadly doesn't apply to humans that well... Maybe we are just too stupid ,be overridden by emotion then be depressed