I’ve been thinking about this, and can certainly see it’s something a character might want to do and therefore we should be able to *try* to kill Ethel after getting her hair. I am less convinced that we should be able to succeed.
The choice we currently have, between power and ridding the Sword Coast of a dangerous being (or killing or letting survive a really cool character!) is a meaningful one, and I think it cheapens it if we can have it all. Plus, it would mean there was no reason, after a bit of meta-gaming, for pretty much any party who wanted to kill Ethel to do anything other than have a go at deceiving her first, which would make the end of the fight more same-y.
I’d be fine if Ethel were given an out. She’s had a bit of time to regroup while you’ve been bargaining with her, perhaps that’s given her the energy for one last invisibility spell/potion and sprint to one of her faerie rings to teleport away.
Hags are known for their deals. Maybe she gets some power over us if we try to betray our deal with her; she can trivially stun/sleep/control us or something like that.
