The novelization of the The Forbidden Planet came out in 1956, the same year as the film. It is credited to W. J. Stuart, which was (according to Wikipedia), a pseudonym for the mystery novelist Philip MacDonald. The 1978 edition I consulted includes an introductory essay by Joseph Milicia, an English professor at the University of Wisconsin, Sheboygan. Unfortunately, Milicia seems not to have had the full story about how the novel came to be written. He does not know the author Stuart's true identity, and he draws some inferences about how the novel came to be written, which may or may not be accurate. In particular, Milicia says that the novel was probably based on an earlier version of the movie script than the final one that was used.
The novel does clarify or elaborate on a number of points that are not delved into so deeply in the movie, and there are a few minor outright differences, although the plot and order of scenes follow the film fairly closely. Narratively, the novel switches back and forth between chapters narrated by Dr. Ostrow, Commander Adams, and Morbius, although Morbius gets by far the least time as the viewpoint character.
At the climax, when the characters face down the monster from Morbius's id, the action is told from the commander's viewpoint.
The breathing was in the corridor. Close.
Something happened to Morbius. He didn't look at Altaira. Or at me. He waved us back. He went to the mouth of the corridor. . .
...
All I know is that there was—Something there. Framed in the rock. Something facing Morbius. Huge, impossible. Looming over him—around him.
Morbius stood like the rock itself. His head was tilted, looking up—
...
I knew the thing that had been facing Morbius was fading.
..
And then it was gone.
But the man kept on standing with his back to us.
His head sank. I could see the strength leaving him. . .
He turned—slowly. And staggered. And came slowly back to us.
Altaira broke away from me. She stood in front of him. She said, "Father! . . . Father!" She was looking up into his face. "Are you all right, Father?"
I moved nearer to them. He said, "Yes, Altaira. Yes." He said, "There's nothing to trouble you now. Nothing."
He swayed. I thought he was going to fall.
I could see his face now. I hardly knew it. It was—it was a good face.
But he was burned out. Exhausted. There was no life behind his eyes.
Based on this, is seems clear that Morbius made a clear decision to destroy the monster, once he really understood what it was. He made an affirmative effort to face the monster down and destroy it, but the process left him too exhausted to do more than guide commander Adams to destroy the Krell machinery and warn Adams and Altaira to get off the planet in a hurry.
This still leaves a bit of uncertainty about the events, however. There are at least two possible interpretations of the passage describing Morbius facing down his monster. One possibility was that Morbius marshalled the power of the Krell machinery using his conscious mind, to a create a force that was even stronger than the monster and could stop its advance; however, it doing so, he expended too much of his life energy to go on much further. A second possibility is that Morbius managed to sever his connection to the machine entirely, so the monster winked out of existence; in this case, it might have been either the plain exertion of the task that killed him, or after having been connected to and supported by the Krell apparatus for so long, he might no longer have been able to survive without the psychic connection to it.
