Monster movies (including alien invasion movies) are easier to accept because they are based on an external threat. The threat in Forbidden Planet is internal, and something evil and destructive that exists within every one of us.
Pretty much all these movies reach a conclusion in which the threat is destroyed (at least until the sequel or the remake), and the viewer can enjoy the sense of relief as he leaves the theater. Not so much with Forbidden Planet, and that’s why it is so scary to those who think about what it means.
Sure, once the Krell machine is destroyed, the ability to project one’s internal evil, backed with almost limitless power, is removed. That gives some relief, but the disturbing reality is that people can always find other ways to project that evil, and given enough time, it can destroy any civilization, just as it destroyed the Krell.
That’s what fascinated me. The ID. It was internal. The monster inside. It left me wondering was it real, can I control it, can I destroy it, and what if it gets loose. It was a great movie for anybody willing to examine it. The human mind is a lot more scary than anything from outer space. The hell man can create it astronomical. Crap. Look what we have now. Chicago on a weekend is enough to make you weep.