Are you sure? You believe there are some people for which the morally right thing to do is to inflect as much misery and suffering as you can, keeping them alive so you can torture them forever, and there is not necessarily even a benefit to yourself or anyone else to doing this?

The negative utility need not be boundless or even monotonic. A coherent preference system could count a modest amount of misery experienced by people fitting certain criteria to be positive while extreme misery and torture of the same individual is evaluated negatively.