While this may be true on an individual basis, it seems abhorrent on a species level. I'll let JenniferRM to put it much better than I could:

Supposing that there was something (vampires?) that preyed on humans and that we had absolutely no pragmatic recourse against these new apex predators (imagine saying "don't eat me!" to a nano-wielding fusion-powered super-intelligence that likes eating humans for some reason) what policy for the consumption of humans would we hope that they adopted? Do we want them to start eating human shaped tofu and relegate humans to zoos and textbooks? Do we want them to bioengineer brainless human-flavored vat steaks and let wildtype humans go extinct? Do we want them to give us adequate human lives and then eat us in our 40's before our flavor goes off? Do we just want them to figure out what maximizes the happiness/virtue/number of vampires (maybe re-engineering themselves to eat algea and wheat and putting every calorie that strikes the planet into the production of this food)? Do we want them to manage wild human populations such that global biomass and biodiversity is maximized? Do we want them to domesticate us via manipulation of our genetics into a variety of types designed to serve specific ends within their economy? Maybe they could offer us some kind of "minotaur feeding treaty" where we managed some of these issues ourselves in exchange for super-trinkets? Lots of those suggestions feel to me like they have good points and bad points... so framing the question this way helps me see that I don't really know what kind of global dietary regime that I really want to bring about with my food choices as an apex predator. I am confused.

Once modified into an appropriate species, I agree with you that each individual murder would be less evil. But the act of modifying an entire species for perpetuity (or the extinction of their species) would vastly overwhelm each kill together...I think. Then again my ethics system isn't really designed to balance such concerns as "permanent extinction (except zoos) of a species" vs "perpetual slavery and torture of a species." I can only say that I wouldn't want that to happen to my species, and I cannot imagine any conscious species desiring such an outcome.