Decent episode.
I think Glenn or Maggie will die this season. Relationships don't end well in "The Walking Dead" world, and Glenn taking the picture of Maggie just set off red flags in my head.
I'd like to think Lizzie (the older girl who lost her father) is the one feeding the walkers at the gate, but when they showed the scene it seemed the person doing the feeding is as tall as an average adult, due to the height the flashlight was being held and where the rat was dangling from.
I think the person(s) responsible for the death of Karen and the other possibly-sick-with-the-flu person is someone we haven't met before. There are so many people in the prison now, it's tough to get a grasp on who is who.
Some fans think Lizzie killed them, using the knife Carol gave her and wanting to eradicate the sickness that led to her father being killed. While I can see that happening, and though we know Lizzie is "messed up," I just don't see a young girl overpowering two adults, killing them and dragging their bodies out to be set ablaze.
However, if Lizzie is the killer, I could see it playing out like the incident between Billy and Ben in the comic book series. Both of them were brothers and, when their parents died, Dale and Andrea began taking care of them. However... (spoilers below, highlight to read)
Ben is a little sociopath and ends up murdering Billy, with a knife. When it's found out, Ben is locked away from everyone else while the adults decide what to do with him. Carl sneaks in to where Ben is and kills Ben. Here's a link to a picture of Ben at his brother's murder scene: http://images2.wikia.nocookie.net/__cb20110218182743/walkingdead/images/a/ac/Billys_death.jpg (that link isn't too spoilerish if you haven't read the white text)
So, I think there's a killer amidst the survivors of Woodbury. The show's writers might be using a couple of characters from the comics to push this story forward, namely
Thomas Richards and/or
Chris.
But there's also the theory that Karen is the Governor's mole, and the Governor somehow still has Karen's son,
Noah. We never see him die, or escape Woodbury, so I suppose it's possible. The Gov somehow reaches out to Karen and uses her son to force her to feed the walkers at the fence, and kill whomever she can on the inside. Karen is the only character seen with a flashlight this past episode... But it could be a red herring.