I disagree, at least in the general case. I believe everyone has a basic responsibility for the well-being of others (insofar as you can reasonably help), and so even if the negotiated agreement is bad I think it's more virtuous to end on good terms. Threatening to quit just because you have a ton of leverage isn't illegal or immoral, but I'd look down on somebody who did it as prizing money over relationships, which to me is definitely a vice.