There exists mods for the last three Elder Scrolls games that let you do exactly this. Morrowind has a mod you can play as a Sixth House member, and even change into an Ascended Sleeper. Oblivion has mods that let you take down the Empire, and Skyrim has mods that let you become the King of Skyrim.
Fable 3 I believe lets you become King and you can be an evil king should you choose, even in the earlier games you could be evil and wield some power (Usually to the extent of being a tyrannical landlord though)
The REAL game you are looking for though? Mount & Blade: Warband. This sandbox game lets you become King of your own sovereign nation and it's up to you how you deal with things, you could go around burning villages to the ground, capturing enemy lords and never releasing them, sell your companions into slavery for a bit of gold, extort farmers for their goods, ransom women found in keeps you capture, and attack merchant caravans.