Pathfinder: Wrath of the Righteous allows you to play as a bloodthirsty demon. You must unlock the Demon Mythic Path to do this. You can learn how to unlock this Mythic Path below.
The Demon Mythic Path can be unlocked early in the game and is the direct counterpart to the Angel Mythic Path. Both are the traditional paths for good and evil respectively.
At the end of the Shield Maze, you will come face to face with a demon. You will have the option to give in to your rage and this will unlock the Demon Mythic Path. Choosing the other option will not unlock the Mythic Path. It should be obvious that the Demon Mythic Path is an evil option.
If you decide not to select the evil option in the dialogue, you will have another chance to unlock the Demon Mythic Path. This occurs when you lead the Crusade army into the World Wound for the first time. As a demon, you can take on demonic forms as well as summon demons. It might seem like an odd choice for the Crusades but sometimes you have to fight fire with fire. Or demons with demons in this case.
For more information on Pathfinder: Wrath of the Righteous, check out Where to find Kaylessa in Pathfinder: Wrath of the Righteous? and How to solve the Gray Garrison Statue Puzzle in Pathfinder: Wrath of the Righteous on Pro Game Guides.