People aren't very good at being utilitarians when there's heavy emotional issues involved even if they are generally good at thinking rationally in other situations. For example, I'm generally a utilitarian, but when I read about this extremely disturbing story I wanted the people responsible to suffer badly for a very long time. And I still do. I don't just want them to die to prevent future harm. I want them to burn. I want them to burn so much that it almost makes me wish there were a vengeful god to torture them. And if I had the choice between simply killing the people involved and making them die slow, agonizing deaths, I'd likely pick the second and them lie to myself and convince myself that that was somehow the utilitarian thing to do.

Humans have a lot of trouble being good utilitarians when the stakes are high.