E. O. Wilson criticized Bend Sinster for choosing the theme of totalitarianism but then failing to engage with the moral gravity of the harms it creates, by depicting dictators as merely nasty and vulgar. I will argue that this criticism of Bend Sinister is wrong because the main conceit of Bend Sinister turns on precisely the moral gravity of the harms of totalitarianism, by way of the familiar idea that it is inconcievable how a just creator could have allowed such harms.
Specifically, I want to propose that Bend Sinister presents a kind of artistically vivid thought experiment analogous to Borges' story in which certain stones fail to obey the laws of arithmetic. Both works describe a prima facie impossible scenario as coherently and convincingly as writerly ingenuity allows. In Borges' case the scenario is that the laws of arithmetic fail to apply to some stones; In Nabokov's case it's that a world containing the harms of totalitarianism should have been chosen by a just creator.
In order to make the latter impossible-seeming scenario feel coherent and plausible, Nabokov draws on our judgements about harms suffered by characters in a novel. There's nothing unjust or morally wrong in making innocent characters in a novel suffer if this best promotes the overall design of the work. So, to the extent that we can conceive of a scenario in which we and the humans around us turn out to be merely characters in a novel (and hence don't have experience pain in any sense that is morally relevant), we can thereby conceive of a scenario in which the harms of totalitarianism are the result of the activities of a just creator.
But is it conceivable that we could turn out to be merely characters in a novel, incapable of experiencing suffering in a way that is morally relevant? Absent the effects of ingeniously deceptive artwork, the idea that we are mere fictional characters and hence don't experience pain and pleasure etc. in ways that are morally relevant seems as flatly inconceivable as the idea that the laws of arithmetic should fail to apply. The result of this is that, in considering both works we experience a kind of imaginative duck-rabit effect, as we bounce back and forth between finding the relevant states of affairs conceivable vs. inconceivable.