This evil king has a supernatural/magical power, which gives him a big advantage over his enemies and allows him to win battles by himself. Although he has never been a king, he now wants to be one, and for that, he defeated a kingdom and took the throne.
He kidnapped to his kingdom a certain elder known by him who has a great knowledge in the world's natural magical energy source, which could be used for technological development.
Now, how could this kingdom grow and develop, i.e. to have soldiers, right hands, followers, considering that every defensive force was killed? Workers and population could be gotten by enslaving people from other conquered kingdoms/villages, but for that he would need soldiers, because it wouldn't be possible to one single person to enslave a whole people (people are slippery). But how?
P.S.: He is evil, but he can pretend not to be so when needed.
I had an idea. Instead of him invading the kingdom and killing everyone, maybe he could get in there, kill the guards blocking his passage, go to the throne room and kill the king, announce himself as the new king, create some kind of agreement with the people and the other authorities, and kill every resistance against him. Would it make sense?