She was crazy to begin with, but the fact that Carol killed Lizzie revealed just how dark the world has become. And it was a bold move by the show itself to not back out from such a controversial death.
You walk outside, you risk your life. You take a drink of water, you risk your life. Nowadays you breath and you risk your life. You don’t have a choice. The only thing you can choose is what you’re risking it for.