Try to explain that to a hungry crowd watching your well-fed family leaving a bunker.

"Oh please, what you're doing is morally wrong. Shame on you!"

I'm not saying that immorality alone will stop many people, but your rationale proves too much, and would stop you from intervening in any immoral act.

>Envy does not justify murder

In a post apocalyptic society it's kill or be killed.

In a sense. The size of the group matters - inside the group you have trusted allies. Two foragers are safer than one; three are safer than two. And so one.