I fundamentally agree with you that publicly-traded corporations are inherently problematic due to the nature of their fiduciary duty to shareholders. That said, I think Costco is clearly the lesser of these particular evils. I have no doubt that given time they'll turn to shit, but thus far (to my understanding) they at least pay something resembling a living wage and have reasonably consumer-friendly policies.

Shopping local could (if even possible) be the more ethical option, but most people are struggling and small grocers simply don't have the margins to generally be affordable. I'm betting very few of them are able to pay their employees anything close to Costco either (if they even care to - plenty of small business owners are greedy in their own right).

Now if only I didn't have a panic attack every time I set foot in a Costco ...