Games like Insider Threat
Games like Insider Threat include Outlast, Poppy Playtime, Visage, Outlast 2, and Slender: The Arrival. We matched them by the tags players give Insider Threat on Steam (Indie, Adventure, Horror, Psychological Horror, and Dark): closest matches first, and among equally close ones the better-known games.

Outlast
Shared: Horror · First-Person · Psychological Horror

Poppy Playtime
Shared: Horror · First-Person · Dark

Visage
Shared: Horror · Psychological Horror · Dark

Outlast 2
Shared: Horror · Psychological Horror · First-Person

Slender: The Arrival
Shared: Horror · Indie · First-Person

SOMA
Shared: Horror · Psychological Horror · Dark

Layers of Fear (2016)
Shared: Horror · Psychological Horror · Indie

From The Darkness
Shared: Horror · Indie · Psychological Horror

Blair Witch
Shared: Adventure · Horror · Indie

Firewatch
Shared: Adventure · Atmospheric · Walking Simulator

The Beast Inside
Shared: Adventure · Horror · First-Person

Garten of Banban
Shared: Horror · Psychological Horror · First-Person

Little Nightmares
Shared: Horror · Adventure · Indie

What Remains of Edith Finch
Shared: Adventure · Walking Simulator · Atmospheric

Among the Sleep - Enhanced Edition
Shared: Indie · Horror · Adventure

Five Nights at Freddy's
Shared: Horror · Indie · Psychological Horror

Amnesia: The Dark Descent
Shared: Horror · Psychological Horror · First-Person

Escape the Backrooms
Shared: Horror · Psychological Horror · First-Person

Amnesia: A Machine For Pigs
Shared: Horror · Walking Simulator · Psychological Horror

Lethal Company
Shared: Horror · First-Person · Psychological Horror

Bendy and the Ink Machine
Shared: Horror · First-Person · Indie

Resident Evil 7 Biohazard
Shared: Horror · First-Person · Survival Horror

Little Nightmares II
Shared: Adventure · Horror · Dark

Inside the Backrooms
Shared: Horror · Psychological Horror · Adventure