Their own subconscious hate and lust for destruction.
6 seconds sound clip from the Forbidden Planet (1956) movie soundboard.
You can hear this line at 01:27:33 in the Blu-ray version of the movie.
Quote context
[...]
- Why haven't I seen this all along?
- But like you, the Krell forgot one deadly danger...
- Their own subconscious hate and lust for destruction.
- The beast. The mindless primitive. Even the Krell must have evolved from that beginning.
[...]