Judging by your bathroom decor, I'm surprised a mere TV programme can have that effect on you, Chuck!
It wasn't a scary/gory episode, it was an unexpected event that threw me big time.
anyone who watches the show, and hasn't seen it yet, leave the blog! LOL Spoilers ahead.
In prior seasons there were two groups of humans, the main group living in a prison led by Rick, and a second group living in a town called Woodbury led by "the Governor".
The governor had major issues, and when the two groups clashed, the Prison Group won. They then took a school bus to Woodbury and invited their survivors to join them in the prison. Two survivors were sisters Lizzie (12) and Mika (10). Each sister had a major fault. Mika was too kind-hearted when it came to people. Lizzie seemed to have a major mental issue, and couldn't always view the zombies as dangerous.
In an effort to teach the group's younger members self defense, Prison member Carol taught them how to use knives and guns for self-defense against zombies.
Last season ended with a major attack against the prison, and the survivors had to scatter to survive. One splinter group was Carol, Tyrese, the infant Judith, and Lizzie and Mika. They survived on the road for a while, and then found a house to hide in while they rested for a few days. While there, Lizzie began to exhibit some strange behaviors regarding the zombies. She believed they weren't always dangerous, and was trying to convince the others they were safe. Carol and Tyrese went out to search for food, and came back to find 10 year old Mika dead, and Lizzie covered in blood. Lizzie had killed her younger sister, in an effort to prove to Carol and Tyrese that Mika would come back, and not be dangerous. She said she wanted to do the same to Judith, the baby.
Tyrese and Carol discussed what to do, and Carol said that Lizzie couldn't be trusted to do the right thing with walkers, and couldn't be trusted around people. For their safetly, Carol walked 12 year old Lizzie into the woods, shot and killed her.