Your "pure utilitarianism" is a complete straw man. Killing someone to take their organs to save 5 lives is utilitarianism to only first order effects. We live

Your "pure utilitarianism" is a complete straw man. Killing someone to take their organs to save 5 lives is utilitarianism to only first order effects. We live in a world that does not terminate after one time step, so we have to consider Nth order effects to calculate utility. For example, the second order effect is other humans' moral judgements. "How horrible, he murdered that man" is a valid moral reaction to have, and this is disutility that must be accounted for in a "pure utilitarian" world view. Third order effects may be the social disorder that results from allowing such ad hoc killings as means to an end, and so on. The only thing preventing pure utilitarianism from being viable is a lack of compute power, and "rule utilitarianism" is a poor heuristic based approach for philosophers without patience ;)