‘You won’t ruin the other woman… the mad one?’ ‘I won’t ruin any mad women, either of them, but it seems I’m going to ruin the sane one: I’m so vile and disgusting, Dasha, that it seems I really will call for you “at the final end”, as you put it, and you, despite all your sanity, you’ll come.
‘The mad one’ is obviously Marya Timofeyevna, whom he’s ruined, and the ‘sane one’ is Liza, who is sick right now probably because of him and whom he will ruin later. We already know why Stavrogin married Marya - ‘moral carnality’ - and he’s done the same to Dasha and he knows he’ll do the same to Liza.
The thing is Dasha is kind of a willing victim in this case, because as Stavrogin tells her, despite her sanity, she’ll still run to him if he calls her. Indirectly, Stavrogin also refers to Dasha when he says ‘the sane one.’
