Anders: I almost killed a girl.
Varric: You've killed two-hundred and fifty-four by my last count. Plus about five hundred men, a few dozen giant spiders, and at least two demons.
Anders: It's not the same.
Varric: Why? Because this one you feel bad about? Maybe that's the problem.
Bolded last line because it's the one I was thinking of. The thing is, my PC's often start pretty "normal." Killing people is morally troubling, they see others as having inherent value simply by being living sapient creatures, etc.. But my thinking has always been that you can't be the same person after all the trout these heroes go through. They will inevitably be warped or desensitized by all the killing. You kill, conservatively, hundreds of people in an RPG. Yet a cutscene pops up and I'm supposed to suddenly feel conflicted? Life comes and goes, it's as simple as that. No need to be hung up over it.
This especially happened to my first Hawke who eventually became "hardened" to life in Kirkwall. He cared only for his own personal principles and those closest to him. I had that guy who was screwing over the Bone Pit money killed and dropped in a gutter, I had my dog maul that burglar, and so-on. By the end, Hawke just didn't care about anybody or anything except what he chose to care about, if that makes sense.