How do you get the bad end with Rat King?
You need to reduce your sanity to 10 and eat him out while have fun with him. Then, there a choice that you can choose between fight your own argue or continue. You need to continue, it will lead you to bad ending.
If you use insane potion to reduce your sanity. You can reduce it to 20. You need to have fun with him once to reduce it to 10.