If you're assuming imagination ~= simulation for an argument, you need to state the assumption up-front. That assumption doesn't hold for humans, nor for anything reasonably imaginable for people in our culture.

> Torturing someone in a simulation is not any different than […] performing lots of arithmetic operations.

Near as we can tell, the universe itself just performs lots of arithmetic operations.

I'm going to give an example similar to:

> If you torture someone then use a time machine to restore him to the initial state, did you do anything bad?

to make a point. If you create a person, torture them, then incinerate the person to restore them to the original state, did you do anything bad? If you're privileging "the present state of the universe, excluding historical events, even when evidence of those historical events is apparent in the present state of the universe", you're taking a very niche ethical viewpoint: one where anything can be undone by killing everyone involved.

> So the only logical thing is to admit that torturing/killing/hunting a species to extinction is bad not because of pain, but because of stealing computational resources belonging to someone else/some other sophisticated computational process.

I'd call this the principle of explosion, but I don't actually see how this can even follow from what you said earlier.

The example with creating a person and then destroying it is different, because after creation it can continue to live independently from yourself. But if you create a person in your computer, that person can exist only as long as you provide the computational power for it to exist. In a way it is similar to abortion, where destroying a fetus is ok because it cannot exist on it's own, and the mother cannot be forced to provide her resources to it.