What is the many but the one? The collective is an abstraction, the individual is a reality. When we adopt utilitarian ethics, or something similar, in our minds we tend to place the abstraction above the reality. This, combined with our flawed ability to analyze ethical situations and predict the outcomes of our actions, can lead to actions being taken based on a utilitarian justification, but lead to results that are undesirable under utilitarian analysis. So to speak, you end up chasing ghosts. There's certain ways in which putting the many above the one is a logically justifiable decision, but it's terrible as a universal.