Oh man, that was beautiful. If this episode would have been a standalone movie, I would have gone to watch it. Things might not have made much of sense, as most of the exposition was cut out and only few lines told what happened to the world. But I would not have minded as what was told made totally sense. There is no need to kill everyone. All life is precious. However, what wasn't made clear is that there is a devil in each and everyone of us.
It's there waiting to come out. Just in our world, a person can live through their life without the evil surfacing.
In the world of TWD, there is no choice, you cannot avoid killing a person. You can try to avoid it but you cannot escape it if you want to survive. Some people like Gabriel, they fight till the bitter end for not taking a life. Morgan, however, we've seen that he only needs to flip the switch and the crazy person bursts through like Jack in The Shining. Or that's what I believe.
This episode was written by the showrunner Scott Gimple if you didn't notice. Also Glenn's credits had been removed from the title sequence.