The first scene of the first episode of the first season opened with a little girl zombie being killed.
A big theme of the show is about what it is to be human, holding on to humanity. That girl had lost all touch with humanity. She was a monster, even though she wasn't a zombie. The show has dealt so much with the human monster, human danger threat being more than the zombie threat.
That girl with all her talk about the zombies wanting her to become one, that they are fine. Even though she wasn't bitten, she let herself be taken over and might as well of been one.
The significance is heightened by her being a kid, because children represent hope for the future. Her sister represented a good hope. She didn't. She destroyed the good hope. She had to be destroyed. Does this mean all hope is lost?
Threats and danger can come from anywhere in this world.
You should look to the story and the themes, not just go, 'character was a kid, automatically is bad' and discount everything else.
This was probably the only interesting episode in this boring second half of a season.