I don't think it's that easy. It's not just about killing the cosette and everything will magically be resolved, y'now. OK, they kill the cosette, but what about the consequences for that? and what's more, there's a demon in her body, I'm sure it's not that simple and easy to kill her...
Because Cosette is one of the strongest demons. When it comes to demons there are rules, which why erez can't do nothing until he has a plan. Keira powers aren't awaken. This is a story with building, it's not just gonna happen but I won't like it is taking a bit to long and it's due to the author writing in romance that makes it drag out for way too long
I won't lie*
Can’t Undin just give the powers to Keira already? Like that would solve her legitimacy and the issue about her mom.
What’s stopping the duke, keira, and the ml to kill cosette? Just kill her already and the problem will be solve.