Abby was raised in an environment where that was expected, so her view of morality is going to be purely based on her living in a bubble. From her perspective, some psycho came in and killed everyone that was engaged in “moral behaviors”. So you can fully understand where she is coming from. Like was said, the story is told from a different perspective from someone with th different morality and life experiences. Joel has seen people get killed for “the greater good” and the corruption and fear that lies behind many of those acts. The Firefly people to Joel were no different than the grunt that killed his daughter or the military personnel that evacuated people from towns and then gunned them down