And what matters even more than loyalty, in this kind of game?

Contracts.

If this is getting ugly, and the company has a product/service that could take off and be worth serious money, your only friend is your lawyer, and your primary assets should words with some signatures underneath.

And if, heaven forbid, we're talking about a cofounder who doesn't have a rock solid contract, the time to fix that is right now, with the help of that lawyer, while leaving immediately still appears to be a significant threat to the business.