There is no such thing as a legitimate king.
There is, perhaps, such a thing as a relatively harmless king, insofar as the constitutional part of a constitutional monarchy can be strong enough to make the king just another celebrity.
There may, perhaps, even be such a thing as a good king, insofar as a person born to royalty can at least theoretically develop a sense of empathy and do more good than evil in the world. If you are lucky.
But there is no such thing as a legitimate king. Royalty as an institution has no business anywhere near government, and people belonging to traditionally royal families do not deserve special treatment due to their pedigree. Existing royal and noble titles should be abolished, and their previous holders subjected to the exact same set of laws that govern all other persons.
The most ironic thing is that anywhere else in the world other than the United States, this position would be called Republican.
It is also important to consider the logical and very real conclusion of this line of reasoning: that countries controlled by absolute monarchies have no legitimate government whatsoever, and should be treated as such. While seemingly radical, as time goes by, this position increasingly seems like the only viable option. It simply makes sense, when you see the world's nations for what they are.
North Korea has no legitimate government.
Large swaths of the Arabian Peninsula have no legitimate government.
Russia has no legitimate government, even though he is not technically a king.
Sweden, Norway, and Denmark have legitimate governments in the form of their respective parliaments, despite the vestigial royalty. But the royalty must go as soon as the political will to do so exists.
Currently, the United States still has a legitimate (though morally repugnant) government, insofar as Donald Trump was lawfully elected to a second term that he was eligible for, although he has grossly exceeded the authority afforded a president. But the country is slowly moving in a direction where, in a few years, that may no longer be true, and we cannot afford to silently let even imperfect democracy die.
No kings in America. No kings anywhere.