I don't think it's inevitable with large groups. Take charities for example. There are very large charities that do very good work, and don't exhibit the kind of fuckery we see in the corporate world. There are certainly bad charities too, but I'd argue those are fraudulent charities run by unethical people.

So what's the difference between a large reputable charity, and a corporation at a similar scale in terms of number of people involved, and amount of money involved? One is nonprofit, the other is for profit. So it's large group plus profit motive that causes the drift toward amorality.