The Walking Dead
Melissa McBride as Carol Peletier in The Walking DeadAMC

*** WARNING: The following article contains spoilers from episode 14 of The Walking Dead's fourth season ***

US horror series The Walking Dead shocked US audiences last night with a powerful, brutal episode that provided one of the hit show's most memorable moments.

Following a mid-season finale that saw the show's core group go their separate ways following a desperate battle at the prison, this week's episode focused on Carol (Melissa McBride), Tyrese (Chad Coleman) and the three children under their protection: Lizzie (Brighton Sharbino), Mika (Kyla Kenedy) and baby Judith.

Having found the perfect place to call home (as perfect as somewhere can be in a post-apocalyptic zombie nightmare world) things were naturally "too good to be true" as McBridge explains about latest episode The Grove.

What we discover is that Lizzie and Mika aren't suited to survival in this world. Both are unwilling to kill zombies, but Lizzie's reasoning takes things a step further – she believes that they're people who just want humans to join them in their eternal life.

In perhaps the series' most devastating and horrific scene, we find that Lizzie has stabbed her own sister to death, insisting that "she'll come back" because she "didn't hurt her brain". She's insistent that the undead aren't dangerous, and holds Carol and Tyrese at gunpoint so they can see Mika change into a zombie. She then reveals that she was about to kill baby Judith too.

Obviously unsuited to the world they live in, Carol and Tyrese decide that Lizzie is also a threat to them and that she must be killed. Carol does so in a moment reminiscent of John Steinbeck's Of Mice and Men climatic scene.

The Walking Dead
The Walking Dead has been one of America's top TV shows since its debut in 2010AMC

Speaking to The Hollywood Reporter about the pivotal, character-defining scene, Melissa McBride revealed her reaction upon first hearing about the events of The Grove, saying she was "overwhelmed and devastated".

She was then asked how this would affect the character as the show continues. "It was something that had to be done in that world and under those circumstances; Lizzie in that world seemed inevitable. It would be impossible for Tyreese and Carol to move forward with Judith, who doesn't have any experience of the world before the apocalypse. It was so devastating for Carol to have to do that."

McBride also said she didn't think her character could have handled the situation any differently. "No, I don't think there was really any other option. There's a lot of nature vs. nurture going on in this episode to look at.

"As much as it broke Carol's heart to have to do this and to realize this had to be done, when they were walking toward the flowers in that scene and Lizzie says, 'You're mad at me and I'm sorry.' You'd think she'd be sorry for stabbing her sister to death but instead she's sorry for pointing gun at her and she just doesn't get it.

"I'm really interested to see how her mind set is moving forward after this. You have Judith and it's like trying to figure out again how do we adapt to this world. You have children now who are coming up who aren't aware of the world before the apocalypse. They'll experience these horrible things and how does that redefine trauma when it's something that's commonplace?"

Another question raised by the episode is what it might mean for the relationship between Carol and lead character Rick (Andrew Lincoln). How would a parent react to the killing of children?

"We'll have to wait and see," said McBride. "There's a lot going on between the time he banished Carol. Even when he comes back to the prison, he turns to Maggie and says, 'There's no time to doubt yourself,' which I thought was great after Carol gives him the watch with how she sees time. There was no time for her to doubt herself when she put Karen and David down. A lot has happened, maybe perspectives are changing, we'll see."

The Walking Dead airs Sundays in the US and on Fox in the UK Monday evenings at 9pm.