While dying of tuberculosis, Ippolit first tries to extort money from and then comes to live with Myshkin. Young, nihilistic, and mostly just very depressed because of his terminal illness, Ippolit reads a long "confession" to the prince and his guests, just before botching his suicide attempt.
You want to know what a fixation looks like? Well, just check out how many times this novel brings up the idea of a person who has to live with the knowledge that he/she is going to die. No, no, not like we all think we're going to die—of very old age, asleep in bed—but actually imminently going to die for some horrible, potentially painful reason, and definitely in the prime of life.
First, we've got those two stories about people sentenced to death that Myshkin tells. They include some extremely graphic end-of-life-fear descriptions, which even in themselves are really overkill, no? But then we also have Nastasya, who pretty clearly realizes that Rogozhin is going to kill her, but still goes to her fate semi-willingly.
And finally, we have Ippolit, the very young man who has the terminal illness of tuberculosis, and whose daily attempts of dealing with impending death we get to witness first-hand. Well, okay, second-hand, though the narrator, but you know what we mean. And you know what's interesting about him? However, unpleasant a guy he is—and seriously, he is pretty irritating—his psychological transformation might well be the one Dostoevsky would most like his readers to take away from the novel.
Ippolit starts out feeling totally entitled, like the world owes him big for his troubles—remember him hanging out obnoxiously in Lebedev's house, harassing his uncle for money? Then he decides: you know what, the world sucks, and nothing matters—and so he becomes a nihilist, rejecting everything and everyone around him and trying to take a totally cavalier attitude toward his disease. This is never more shocking than when we see him going to a similarly nihilist doctor, who says, all casual, "eh, you'll die pretty soon."
But finally, as we learn from his confession, the more sick he becomes, the more Ippolit starts groping for some kind of meaning in the universe, and trying to find some connection with the divine. Suddenly, he realizes the contribution he could make to the world if he lived. (Remember the long story of helping the guy who lost his wallet get his job back?) This is how he ends up feeling a little more at peace with the idea of death.
But what is great is that although his beliefs change, his core personality doesn't really transform in any way. He's still a nasty piece of work to the Ivolgins, particularly Ganya, whom he totally lays into before leaving the Ivolgins' house forever:
"I will proceed to explain why I specially wished to make [Ganya] look a fool. That my motive power is hate, I do not attempt to conceal. I have felt that before dying (and I am dying, however much fatter I may appear to you), I must absolutely make a fool of, at least, one of that class of men which has dogged me all my life, which I hate so cordially, and which is so prominently represented by your much esteemed brother. I should not enjoy paradise nearly so much without having done this first. I hate you, Gavrila Ardalionovitch, solely (this may seem curious to you, but I repeat)—solely because you are the type, and incarnation, and head, and crown of the most impudent, the most self-satisfied, the most vulgar and detestable form of commonplaceness. You are ordinary of the ordinary; you have no chance of ever fathering the pettiest idea of your own. And yet you are as jealous and conceited as you can possibly be; you consider yourself a great genius." (4.2.67-71)
He might well be right (at least the narrator certainly agrees with him—check out the "Ganya" character description for a comparison between what Ippolit thinks and what the narrator tells us), but still, dude, that's way harsh to say to someone's face. Why do you think does Dostoevsky give this character a spiritual transformation but leave his unpleasantness the same? Does this make the spiritual stuff more or less realistic? Why?