I think I see where you're going with this, but your presentation actually leads somewhere else entirely; you discuss your point in the introduction and the conclusion, but you invest the majority of the weight of your argument into a place where your point is nowhere to be found.

Your argument, as presented, seems to be along these lines: Suppose there is one person, and one person is tortured. That's really important. Suppose there are a billion people, and one of them is tortured. That's not very important.

What I think you're getting at is more along these lines: Suppose there is one person, and one person is tortured; that is extremely important to that person. Suppose there are a billion copies of that one person, and one of them is tortured. Even a slight benefit arising from a decision leading to that torture may outweigh, by virtue of the fact that the benefit has been reproduced a billion less one times, the badness of the torture.

In other words, your presentation conflates the number of people with the importance of something bad happening to one of them. You don't discuss potential rewards at all; it's just, this torture is happening. Torture is equally bad regardless of the percentage of the population that is being tortured (given a specific number of people that are being tortured, I mean); we shouldn't care less about torture merely because there are more people who aren't being tortured. Whereas your actual point, as hinted at, is that, for some group that gains utility per individual as a result of the decision that results in that torture, the relative badness of that decision is dependent on the size of the group.

Or, in other words, you seem to be aiming for a discussion of dustmotes in eyes compared to torture of one person, but you're forgetting to actually discuss the dustmotes.