The fact that the monster is termed the id monster, the cultural currency of Freudian ideas in the US in the mid 20th-century when the film was made, and Dr. Morbius' line "it's an obsolete term, I'm afraid. Much used to describe the elementary basis of the subconscious mind," suggest that the monster is not a rational actor in the sense of a human being of sound mind (leaving aside the fact that humans of sound mind are not actually Homo economicus :). The id monster is a being of emotion and rather immediate and reactive emotion at that... from Wikipedia the idid "is the source of our bodily needs, wants, desires, and impulses, particularly our sexual and aggressive drives."
So the id monster was these impulses and drives made tangible and divorced from the integrated reasoning and moral parts of Dr. Morbius' self. If the id monster reasoned rather than reacted, it would not be the id monster.