I don't remember which show, movie or book I'm thinking of--maybe Dexter?--but I remember there being a theme of bad guys letting their victims get away once in a while. The conclusion was that bad guys did it A) to retain control and B) to reassure themselves that they're not All Bad.

This is definitely the case with Eragon here; he doesn't want to be a mindless killing machine, since he's the hero. Sparing those parents was the moral reassurance he needed in order to go on massacring more people.