Utilitarianism sets out to have "the greater good for a greater number of people," but what if someone's definition of good is committing something like mass genocide (something on the lines of the Holocaust with the belief that it causes greater good for a greater number of people ). How is "good for a greater number of people" defined according to utilitarainism, considering the pleasure aspect.
Utilitarianism is driven by "what is useful or designed for use" (Webster's definition for utility). While mass genocide might be useful for those who remain, it is not beneficial for those who were killed--was genocide the best option for all? It might have been the easier option, but not the best one. Utilitarianism must struggle with temporality--what's useful now might not be useful later. The best form of utilitarianism not only pursues the most useful option, it considers "the greater good for a greater number of people" over time.
Utilitarianism considers the interests of all humans equally, but there are three distinct camps when it comes to useful. Happiness or pleasure led the way (Benthem 1780), until morality and ethics jumped in (Mill 1861). Sidgwick (1874) tried to add reason to the mix, but now we have the benefits of science that show us pleasure is biological and morality is causal, so useful is Determined. As long as these three camps remain, defining useful depends on which one feels better, is reasonably good, or has already been determined.