This show is about a lot of things, but to me, what it is at its core is an analysis on humanity and what a broken, apocalyptic world does to people. This has to be one of the darkest, most shocking things in the show up to now and I remember not being able to sleep after watching it when it aired. Having to put down a child because she's unreachable is so messed up.. Incredible writing and acting. I think Carol had to tell Tyreese about Karen after all of this because it was the last straw on her conscience.
I also love how this season sets up mysteries (like the dead rats at the prison) and slowly builds up to the answers.