You can end up utilitarian either because you're a psychopath and don't have the special moral module - in which case you default to general purpose reasoning - or because you're very philosophical and have a specific preference for determining moral questions by the same logic with which you determine everything else, thus deliberately overruling the special moral module.

This is an interesting interpretation. The study's authors seemed to suggest that the psychopaths et. al were getting to their answer via a very different route than the thoughtful utilitarians. Your suggestion is more intuitively appealing to me - there is to be expected a larger set of common answers between psychopaths and utilitarians if they are both using reason to answer these questions.