There are several ways to make an evil guy more believable.  One way was suggest in post 2, make the character a simpleton that accidentally creates bad things without really being bad himself.  Another way is the give the bad guy a motive.  Many times we really dislike the evil character until we start to find out why they are really evil.  Sometimes something has happened to them that we can sympathize with.  Perhaps the king stole his girl or perhaps the royalty hurt his family.  We can be a little more understanding of someone who does horrible things if they have a reason for doing them rather than just a blanket sociopath.

If your evil character wins over a friend of the king, it will make him more evil while also opening up some plot options for you. The friend could betray the king. The king could have to save the friend because Mr. Evil has tricked him into doing something dangerous...

My favorite ultra-evil characters always have some allure, like Hans Gruber inDie Hard.He's an evil terrorist, but very charming, well-dressed, and personable.  Often the best evil villains are charismatic or persuasive, like Darth Vader ("You don't know the power of the dark side..."). 

The "evil guy" thing has been done over and over again. Characters who are "pure evil" just aren't very believable. It's possible to create a real character who causes evil but isn't all bad. Try to make your evil guy at least a little sympathetic to the character.