What makes robbing someone or stealing groceries any worse than a lion killing a zebra for food?
There’s no moral aspect to a lion killing a zebra; only humans “do” morality. But the fatal flaw in your thinking is that the human you’re robbing is a zebra to your lion; in fact, he’s another lion — and part a big pride of other lions. If he’s too small or weak to defend himself, the other lions in the pride will not only take back what you stole but will maul you so badly that you will never try it again. And they won’t care all that much if they kill you in the process, or leave you so broken that you’ll never even be able to take down a zebra again.
As to stealing — a better parallel would be that of a hyena’s behavior around lions. Too cowardly to act directly, hoping that it won’t get caught while sneaking up on something it knows it doesn’t deserve to have, and only attacking directly when it knows it’s much stronger, or has its pack around it. Not much there to be proud of.
There can be good reasons — extreme ones — for serious violations of the social compact. But the typical person who does it is a lot more akin to the hyena than to the lion.