he sometimes saw or felt some sort of evil being beside him, mocking and ‘rational’, ‘in various guises and with various personalities, but always one and the same, and I always get angry…’
He's haunted by the same thing as Ivan Karamazov's Devil. When this was casted out from him he, he could not take it (we may see why later); he killed himself.
I do not think this chapter necessary to make a conclusion about why Stavrogin committed suicide because Dostoyevsky has written the rest of the novel in accommodation for this missing chapter. Still, it gives a lot of insight into Stavrogin.
