My current “thing” is the Wars of the Roses, which illustrate a lot of interesting problems with the concept of loyalty. If you decide to support something — King, person, cause, country or whatever — and later decide that was a bad decision, how can it be a good thing to continue to act on the basis of that bad decision?
I can see why the King thinks your loyalty to him is a virtue. It’s certainly makes his life easier to know that his supporters will back him no matter what he does. I can see that it makes your own life easier if you can make a decision and then forget about it, to not have to constantly reevaluate.
The only case I can see when it might be the ethical thing to do is when resisting the temptation to change only because you want to be on the winning side, especially the temptation to try to make both sides think you’re loyal to them until one of them wins. And even then, I could argue that you have a higher duty to keep yourself, and others you’re responsible for, safe.
But what if the King has changed and is doing evil things? What if the situation has changed and the King you supported, through no fault of his own, is no longer the best person to meet the needs of the country? What if you have matured, and your greater knowledge makes you realize it was naive to make the decision you did? In all these cases, if you continue to be “loyal” after you realize continuing your support will do harm, how can that be a good thing?