I think I agree with your conclusion. But this:

to increase utility we should simply kill off all the rich, and let the poor multiply to take their place (continually bumping off any of the poor that gets too rich).

requires you to assume that the US or "the rich" have no relevant chance of producing vastly happier people in the future. This seems stronger than denying the singularity as such. And it makes targeted killing feel much more attractive to this misanthrope.