Is stealing from someone like Walmart, who post record profits every year, pay employees so little they are on food stamps, engage in union busting etc etc really immoral?
Yes.
So Walmart profit is more important to you than people not starving?
Walmart's profits have no bearing on the fact that stealing is immoral.
Calling your personal morality a fact seems bit over the top.
Walmart has paid over $1.4 billion in wage theft settlements and fines.
And that also is immoral. However, that does not make it moral to steal from them.
If you want food, robbing the very people bringing in food is worse than robbing random strangers.
Walmart is not people, Walmart is a billion dollar company and it is not bringing food, it is selling it for profit. Stealing from a company is a matter of balance sheet, while stealing from people means a real someone (as opposed to a abstract corporate entity) is actually going to miss something.
Moreover, Walmart would throw the food away anyway, if nobody bought it.