Contrary to what most people think, being a King doesn't make you the supreme ruler. A quick game of Crusader Kings would enforce that. Here's what I would do
Define, loosely, the political structure of his realm and his obligations. A king has obligations to his people; he can't just take everything out from the treasury because he has troops to feed, fiefs to upkeep and his castle will require maintenance.
A king who holds all the power is bound to have unhappy subject (hence feudalism). He has to delegate power; and if he does, and if due to his actions, he become a unpopular king, his subjects will rebel.
An evil kingdom sounds like the perfect plot hook to throw all sort of intrigues and political power struggle. He may become the target of various disgruntled nobles, and he is bound to be outnumbered.
- An evil kingdom may have certain rites or ceremonies that expect the king to prove himself to be strong and capable. Have him undertake dangerous tasks (such as raiding another evil kingdom) which, by weight of tradition, will only allow him and a small number of troops to go along.
However, the biggest question is does the PCs like the direction the game is going, that you are shifting to more political power-play and intrigue. If they don't here are some ways:
Have the king-PC become a NPC, and roll up a new character of the same level. The character is now the king's Champion. If the other PCs agree, the King-NPC are now the center of the campaign.
Have the king-PC, to secure his domain, give important titles to the other PCs, so everyone is more or less on the same level. However, the tone of the campaign will change - it's unlikely they will all want to descend into dungeons for loots and treasure.
Another way is to discuss with the your players how to proceed - if they are still fine with the usual style of adventures, work out something together. Me, I will just go with the 'Trial of the Kings' idea