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.
This death was shocking made for the fact that viewers saw the reveal the same time as Daryl. Merle ended up a walker, but he still managed to do one last good deed in taking down some of the Governor's men. You even secretly wished he lived.
Lori's death was a true surprise not only because she went into labor after a walker attack, but Carl then was forced to kill her. The fact that it took place so early in The Walking Dead Season 3 made it even more surprising and revealed that truly no one was safe on the show.