The Apartment Predator

The Apartment Predator is the unofficial name for an unseen serial killer and pedophile who had lived in the Apartments prior to its current state.

Overview

At this time, Simon never encounters him, only finding his "secret notes" scattered throughout the complex. It is possible that the man with the shears in the video recording was him, or the man who was killed, though little evidence supports either speculation, although the clothes worn by the corpse in the bathtub, and the corpse's facial appearance are a match for the person in the video, and it is unlikely that there would be 2 murderers in the apartments. The Apartment Predator was likely responsible for the presence of the particular enemies in the area, notably the Children. His apartment is filled with garbage bags, and contains the "Picture Room", likely where he killed his victims.

Notes

“	3 little kiddies playing in the park, Said the first kiddie, let's go home before it gets dark. The first two went on their merry way home. The third stayed playing. All alone. Come here little child, come to me! The bushes whispered softly. They say curiosity killed the cat...	”
	— My Secret Note 1 (Third Floor, In Hallway)

“	Those little kiddies are so cute. They look so lovely! I just want to touch them... But then those parents come... That's why I have to kill them!	”
	— My Secret Note 2 (Predator's Apartment, Second Floor)

“	Now... As I need to kill some kiddies, I stuck the elevator in the hall. So only the personnel can access it by a secret code. I guess no kiddie will find that out, so they will just have to take the stairs.	”
	— My Secret Note 3 (Outside Roped Off Apartment, Third Floor)

“	This one was a good one. Very nice to kill. Easy to cut with my knife, like cutting food. (NOTE: Only available when you take the elevator to the forth floor. Once you climb down again with the rope this note is no longer accessible).	”
	— My Secret Note 4 (Forth Floor, In Hallway)

Relevance to Simon

It is possible that the Apartment Predator represents one of the many demons infesting Simon's mind, "preying upon" his innocence and roaming around his own "home" (either the apartments or his mind). It's also possible that the predator was an actual being that Simon has met in real life while living in those apartments.