It doesn't seem that there could be a utilitarian argument against murder that would be applicable if there were only two people. That is because, if only one is left after the murder and the murder is quite happy about it, then, such a murder would be for the greater good of all who are left. Or maybe the would-be victim's utilitarian argument against being murdered would amount to "Think of how lonely you'd be if you got rid of me!" An even more compelling point could be made re murder-suicide.
Commentary and discussion regarding science, faith and culture by Leo White