Um, no. If, in the post apocalypse and only then, someone tried to kill me and I caught him instead, I can freely do anything I want with him. He forfeited any claim to humane treatment by trying to kill me. His reason is irrelevant to me, since if he had succeeded I’d be dead and not very impressed with his good reason for killing me.
As to why we constructed this morality concept, I can’t prove it not having been there but here’s my theory:
At first humans lived in small family groups, possibly extended family(cousins, etc). That was fine without morality, might makes right is the only rule you need.
Then at some point larger groups started forming, groups that had no family ties between subgroups. Therefore these people had no personal bonds to regulate the violence, personal bonds that restrained family groups from killing each other. Dear old Dad isn’t going to kill off Junior under most circumstances, so they’re mostly cool with each other. You’ll note that this does not involve moral considerations but only what allows the family to procreate and live on. If dear old Dad has many sons he might well feel free to kill some of them as he sees fit.
However, what happens when dear old Dad is challenged for authority by some guy who is the head of his own, separate, family? Dad may feel he needs to kill this guy and his entire family. This could splinter the larger group, endangering the survival of all. Something has to be done to prevent this. Rules, morality, are needed to allow these larger groups to flourish. So there you go.
Post apocalypse though, we are back to small ‘family’ groups. In the Walking Dead world it’ll be years before those larger multifamily groups have to be formed. In the meantime, morality as we know it now will be not only be unnecessary but interfere with survival. For example, when Rick et al run into that guy in town, the one on the roof shooting at them, they were faced with a choice of what to do about him. Rick tried to do what was ‘right’ which was dumb. Rick’s ‘good’ actions haven’t helped at all, they’ve merely complicated all decisions he’s made and in some cases led to preventable deaths.