I don't expect people intentionally erode their instincts, though. I mean, there are people who will work to fix their fear of spiders or flying or whatever, but I don't think there are people out there who intentionally work themselves up to killing people. Well, outside the military. That doesn't mean that it doesn't happen as a result of something else. I can certainly believe that there are petty criminals who slowly work up to being murderers as their qualms wear away. But (and I'm probably wrong) I read what you said as someone doing it intentionally.

There are definite frequent benefits to overcoming some of those instincts, whereas I can't see the frequent benefits of overcoming the aversion to killing that would come at a point where you hadn't already overcome it, unless you're in the mob or the military.

What I'm getting at is that I don't see a reason that someone would start down that road. Their instincts would tell them not to, unless those instincts aren't there to begin with, in which case no "progress" is necessary.