It's simple, as far as Mae is concerned. She remembers the state of her own world before she came to Wonderland. And sure, she understands power vacuums, and there's the chance that some other equally terrible Mirror could take over in the Queen's absence. But could it really be as bad as it is now? Especially if they found a way to get rid of whatever lets the Queen simply Unmake people.

Killing isn't objective. Everything that happened in the mine drove that home.

"No, but we don't know if the denizens have to follow the same rules. The Red Queen was just talking about how Bill is gone. He 'went inside' or something. Even if it's not the same, I bet she can still be like, permanently taken out. If it's all really chess, I mean--queens can get knocked off the board."