Stansom's back door is the back door, not the front door.
As soon as you go in and open the first door with the key, you will find that the mayor is running away, getting into a door and disappearing.
Go in through the door where the mayor disappeared and kill all the bosses and priests of the psychic tower. If you don't kill them, you can't go into the slaughterhouse.
5. Walk into the slaughterhouse and get rid of the strange things slowly. Don't rush in, or you will be foolish to be locked up in Tully.
6. I wish you an early DK horse.