Thanks for a very tough question, but one which I hope I can provide something like a useful answer for.
BOTH. Lets look at the life an death of one Theodore Robert Bundy, otherwise known as the famous American serial killer and death row heart throb, "Ted Bundy."
Nobody has ever known who Ted's biological father was-and it's widely speculated that his grandfather was also his father. If that sad case of incest is indeed true-then Ted is a perfect example of how incest creates sociopaths.
As for my answer being "both," I think that gene pool inadequacies combined with horrific parenting can also produce the same.