In S04E03 of The Walking Dead, Isolation, Rick discovered who murdered two of the sick people in prison. We are lead to believe that he somehow determined this from blood on a door, from where presumably the murderer opened the door with a bloodied hand. How did Rick make the connection that this was probably their hand?
It was a combination of 2 things:
- The size of the hand print was smaller than Rick's hand, which means the person has a smaller hand.
- When Rick saw Carol was willing to go to great lengths to ensure the safety of the group, he realized she would be willing to kill too. This was due to Carol heading outside the fence to fix the hose that is the group's water supply. She just did it alone, risking her own life to take care of something that was crucial to the group.
He didn't know for sure. He made a guess based on the evidence. Whether his guess was correct or not, remains to be seen for sure.
This is just a theory, but I wanted to put it out there as a possibility. Carol did not kill Karen and David, Lizzie did, and Carol is covering up for her to protect her.
Possible evidence supporting this theory;
- The small bloody hand print on the door was low (Rick had to bend down to see it)
- Carol had trained Lizzie how to use a knife
- Lizzie treated walkers as friends (e.g. she had named one 'Nick') and was upset about the group killing walkers at the fence. She saw Karen and David killing walkers there (possible motive). As a side note, its also possible she was the one feeding the walkers at night.
- At the beginning of S04E04 a scene is depicted where Lizzie insist that she is strong, and it shows her carrying a knife Carol had given her. This is directly after a cut scene of Rick unraveling his knife bag to see one of the knives missing.
- Lizzie was in the sick ward with those killed. Note, she also had somewhat of a "fake" cough to get in there (albeit, that may have been poor acting).
Re-watch the beginning of S04E04, where Carol is talking to Lizzie, and see if you are not convinced. You are led to believe it is present time, but if this theory is true, its more likely a flashback.
What Keen said + Carl told Rick about Carol teaching knife skills to children in secret so they can take care of themselves in an emergency.
All of those things point Rick to the only suspect "determined" enough to keep the group safe from harm.
He establishes the motive very quickly. With that in mind he than mentally crosses people of a list. Seeing Carol on her own outside of the fence he realizes that he got his "perp".
Adding to what's been said, the first visible indication that Rick suspects Carol is in S4E3 around the 11 minute mark. Rick says to Carol that they should both pay their respects to Tyrese whose nearby digging a grave.
Carol coldly says she'll bring up the water first. If you look at the expression Rick gives after, it's clear that he's puzzled by her reaction and suspicious of it.