According to the Spooky's Jumpscare mansion Fandom which can be found here:
https://spookys-jump-scare-mansion.fandom.com/wiki/Endings#B...This achievement is unlocked by getting the
Bad Ending. To do this, you must accumulate 20 "points." You can get these points by swinging your axe at certain specimen. Swinging your axe at the cardboard cutouts is 1 point, at the deer is 2 points, and specimen 2, 3, 6, and 11 are 3 points.
I recommend swinging your axe at the cardboard cutouts as much as possible, as well as specimen 6 (ben the merchant) chases after getting your axe.