They scared me as hell too! I guess nowadays we're more desensitized to violence, but at that time, it seemed too gruesome for me.
- The guillotines in PoP.
- The death animation for victims of the TOZT Napalm Unit in Marathon.
- The "red alert" warning combined with Kestrel fighter escort launching sound in Escape Velocity.
- The wolf 4 screens left in King's Quest 1 (I played this at about age 4 and did not comprehend the idea of going somewhere else. I just kept trying to outrun it)
- Ampire Bot in Robot Odyssey
- The entire Chiller[0] arcade game. I still sometimes have nightmares about it, and I've only ever walked by it once.
They both turn my stomach just thinking about them.
The butcher in Diablo 1, filled with a room of dismembered corpses.
Aztec Challenge came close in creating tension, but not enough to feel scary.