Rather than going intentionally out of your way to be 'evil' throughout the game, just go for Enlightened first. Get a majority of the moral points (the exact number isn't known for certain, so best to get as many as you can), and if you don't get the Enlightened ending you'll get this one. If you do get Enlightened, just click 'continue' when it gets back to the main menu. Once you finish the vision again and are looking at the Dark One and the target locator, just shoot the Dark One instead of the target locator to instantly become 'evil' and unlock this achievement.
I elected to record a full playthrough getting nearly every possible Moral Point in the game, as well as locating all Diary Pages and pointing out various other achievements along the way. It's a long video, but if you'd like a good way to follow along as you go to knock a lot of things out, including these pesky Moral Points and Diary Pages, check out the video below. It demonstrates using the 'instant evil' method to turn the Enlightened ending into the bad ending by shooting the Dark One starting at
1:54:49 mark.