Without getting too analytical, the sort of films that Cyber-Liberty and Smokin' Joe are referring to might be defined as the "gothic horror" genre which often uses grim/dark/strange surroundings, implied or impending violence along with sudden graphic representations - like Mary Shelly's Frankenstein novel and Edgar Allen Poe's work. The original feature film "Alien" directed by Ridley Scott and his sequel are also in that category.
The sort of elements that you folks are referring to are fairly sophisticated for many audiences, and not for everyone. That being said, Forbidden Planet broke new ground by introducing psychological concepts into SF /horror. In F.P. the Freudian ID and a classic Incestuous Daughter-Jealousy theme appeared with Dr. Morbius' unconscious rage (because of his daughter's interest in the Captain) setting off the violence of the Krell machinery. That's fairly heady stuff for a main-stream audience. Yet it was masterfully executed by the director/screenwriters.
A good director will tap into the imagery that emerges from the unconscious in what CG Jung calls archetypes - symbolic representations of themes, fears and desires from most primal levels of consciousness.
Movies like "Jaws" and the "Creature from the Black Lagoon" also tapped into those gnawing, deep fears that we all normally push to outer regions of our awareness. Unseen monsters lurking in the deep water, or in darkness, or in deep caves /caverns. Generally water/darkness are symbols in the psyche for the unknown and the unconscious, which is why they can be so evocative when used in horror films.
When we don't know why certain themes or images trouble us, often it's because the images have touched on some classic archetypal theme that goes to the foundations of awareness.
