Explaining 'Monsters from the Id' explains Forbidden Planet's greatness.
You know a movie is great when it gets better as you grow up and wise up.
When I was a kid, "Monsters from the Id" went right by me. Then, as I grew up, re-watching the movie every chance I got, I began to understand, and the phrase started to make sense.
The Krell had built a machine that transformed thought into material. The machine would read a thought and create it instantaneously, "without instrumentalities," as Morbius put it. To accomplish this, enormous energy must be expended. That was was the purpose of the gigantic Krell power plant: to feed the machine.
Now, conscious thought can be controlled. A benevolent consciousness will never use the machine for evil. But what about the subconscious, particularly the Id -- the beast in us all? This is what the Krell did not reckon with. It is what killed them off, in some horrible orgy of mass murder.
Morbius's problem was Freudian. He loved his daughter, but the latent sexual attraction of a father for his daughter made Commander Adams a rival and therefore an adversary. Morbius's subconscious antipathy harnessed the Krell machine, which dutifully created a monster of Morbius's Id to destroy Adams and all the other men who were potential rivals.
It was the same lethal harnessing of subconscious hostilities amongst the passengers and crew of the Bellerophon -- or, horrible thought, by Morbius against them -- which caused their demise.
Don't know if others will agree with this, but it works for me.
By the way, the 7.7 IMDB score Forbidden Planet gets is a travesty, a reflection of the mediocrity of the audiences who don't get it and move on. 8.7, which the slicker but inferior Star Wars and the Matrix get, would be far more fitting.