I think you're interpreting Bostrom slightly wrong. You seem to be reading his argument (or perhaps just my short distillation of it) as arguing that you're not currently torturing someone, but there's an identical section of the universe elsewhere where you are torturing someone, so you might as well start torturing now.

As you note, that's contradictory - if you're not currently torturing, then your section of the universe must not be identical to the section where the you-copy is torturing.

Instead, assume that you are currently torturing someone. Bostrom's argument is that you're not making the universe worse, because there's a you-copy which is torturing an identical person elsewhere in the universe. At most one of your copies is capable of taking blame for this; the rest are just running the same calculations "a second time", so to say. (Or at least, that's what he's arguing that Unification would say, and using this as a reason to reject it and turn to Duplication, so each copy is morally culpable for causing new suffering.)