Only watch The Predator after you have finished the main 20 episodes. It does not contain new plot resolutions, but it offers a fascinating character study of the antagonist’s daily life. It is essentially a "horror montage" that validates the fan theories about the killer’s hidden clues.

If you knew your child was born with the genes of a serial killer, would you still have them? The "Head Hunter" Legacy:

The show’s title, "Mouse," refers to the lab rats used in psychological experiments—hinting that the characters are subjects in a grotesque game of nature versus nurture.

Mouse: Thrilling, Emotional, & Exhaustive (Review) : r/KDRAMA

Fast forward years later. A series of gruesome murders grips a quiet Korean town. We meet Jung Ba-reum (Lee Seung-gi), a kind-hearted rookie cop, and Go Moo-chi (Lee Hee-joon), a detective haunted by a serial killer from his past. As they hunt a predator known as the “Head Hunter,” the line between good and evil blurs until it disappears entirely.