I once had a discussion with someone, and because I felt like being a prick, I argued that it would be permissable for a group of 4-5 persons to collectively own a slave. Provided that they fed the slave well; it freed them from a lot of work and would increase their happiness; whereas the slave would get used to his status sooner or later and his unhappiness would be less than the happiness it would provide to his masters.

However utilitarians would argue that the sum wouldn't add up, and that slavery (and large scale murder, like you propose) would always cause more suffering than happiness. One reason cited is that a lot of people wouldn't be able to enjoy themselves if their welfare was based on slavery, genocide and whatnot.