according to Svidrigailov, why is raskolnikov now suffering?

I'm not sure exactly which chapter you mean in part six but from a character standpoint Svidrigailov is much more rational if not fatalistic than Raskolnikov . Svidrigailov might be villainous but he knows that he can't simply will people and circumstance to turn out the way he wants. Raskolnikov believes that if he tries hard enough, he can change circumstances. He has more utilitarian vision of the world where his actions are done for the greater good. This is why Svidrigailov thinks he suffers so. Svidrigailov feels that Raskolnikov can't accept the realities of what he has done and who he really is.