At this point Myshkin finally pulls out the letters from Nastasya that Aglaya gave him and reads them.
They are, um, well, pretty much super crazy.
The main ideas in the long, rambling letters are:
(1) Myshkin is a being of perfection and light, and so is Aglaya, so they belong together.
(2) Aglaya is so perfect that she is probably not even human and thus incapable of hatred or love.
(3) Nastasya is so far below Aglaya in every way that it's not possible that these letters are even mildly humiliating to her.
(4) Nastasya is pretty sure that Rogozhin has hidden away a razor to kill her with.
(5) Nastasya is rejecting the world and life.
So yeah. This is about one foot out of the mental institution, no? And yet, to Myshkin, this all makes perfect sense with everything he already knows about the situation. And yeah, what she writes makes pretty good psychological sense—she is someone with zero self-esteem and complete will to self-destruction, so she sees Aglaya as her polar opposite, but also in some ways, the woman whom she could have been.
Anyhow, Myshkin has been reading these letters and wandering around, and he realizes that he accidentally walked to the Epanchin place. Of course. Like a moth to a flame.
They are all in bed already, and he leaves—only to run into Nastasya for reals this time, yo.
She is still pretty crazy and raving, but she does promise to stop writing letters to Aglaya and to go away like she promised.
Then Rogozhin comes to take her away, but chats with the prince first.
He's well aware of the letters also, and chuckles at the whole kill-her-with-a-razor line. Yeah, that's a good one. Then he says that he's leaving the next day too.