How to Get The Evil Ending
After killing the beast, save your game, don’t do just a quicksave.
Talk to the Abbot, your choice here will start a series of events leading to either the good or the evil endings.
To get the evil ending, tell the Abbott you will murder him to keep the secret quiet so you can partner with the Governor. You will enter a huge fight with Flesh Eaters, Fire Gargoyles and Ice Mutant Dogs, get ready. or… you can Recall/Town Portal out of there and escape the fight.
After you get back to the city of Eldfell, talk to the governor and he will give you an axe and a maul.
Use the maul to repair the Thule bridge, which is to the far north of the city near the Watcher’s house.
With the hammer equipped, just walk west until the end of the bridge, the effect should trigger in one of the 3 bridge spots.
Go and talk to Mersant Orasare at the port of Eldfell, the achievement & ending is yours!