"He's a bad guy so let's kill him!" is NEVER justified even though it continues to this day.
Can I ask why? Is it because of your morality? And what defines that?
In other words, finish the following sentence:
Killing a person who committed crimes that are unforgivable is wrong because _________.
The reason I ask is because it is really the central core of my next book and I had to really dive into what is "moral" and what isn't.
In the simplest form of what I came up with, morality is a cultural ideal, not an instinct. I also believe it has religious roots. Take those two things away: the instilling of morality from your parents (ie "don't do that, it's wrong") and the religious repercussions for doing so (ie "you will suffer for all eternity, not just until you die"), and what is left to prevent it? Without those things holding you back, when someone is a threat that will never go away until you kill them, why would it not be a clear choice?
Keep in mind, this is a Post Apocalyptic world. If you were religious (at least Christian based religion) you were left behind because this was, by definition, the Apocalypse. The ending of humans on earth. If you are left, you are Forsaken. Sorry, but regardless of what you do now, you no longer have a place in Heaven.
And even if you never believed in an eternity, the culture that restricted you from committing murder no longer exists. In fact, nobody would blame you for killing someone because chances are every stranger is a threat. There could even be a good argument that it is necessary. You let him go and he will find more people and tell them you have food, shelter, weapons, etc. and they will return to try to kill you again.
I think the episode does a good job of diving into this issue. Morgan's moral base was gone, completely. He had devolved into an animal that had one purpose: kill everything that gets close. There was nothing he cared about enough, including himself or his soul, to tell him there is something better, and nothing he did made him feel better. Then he meets a man who shows him a way that he can feel better about himself. With no other stimulus and no consequences, would you live your life in a way that felt better, that allowed you to sleep better at night and have positive emotions, or would you choose to keep living a life that was empty?
When you start to look at the bigger picture, you have to ask what makes these people tick now that that moral base from culture and religion is stripped away. The culture that arises from the chaos will not be influenced by what society used to be, it will be born first of necessity and then from tradition. "Why do we kill people who commit crimes, dad?" "Because that's what we do, son."