I agree that in this case the parents were probably somewhat to blame. From what I've read about them and seen from them they seem a little radical and absent as parents. At this point I don't think they are criminally to blame for their kids actions but probably had something to do with the formation of their kids initial belief and value system, however all that comes from very limited knowledge of their situation. A lot of factors go into a persons psyche including parenting, socialization, culture, drug use, education, etc. I also didn't put much into the parents initial reaction to their kids actions either, they seemed to react with a very normal denial and fear that any parent would react with given such dire news.
I would agree with Vanilla Gorilla that you should use caution in who you place blame on and how much blame and punishment you wish to dish out before the facts are known. Ive seen a lot of troubling mob like mentality and hate associated with this story, which is probably associated with their religion and the gruesome nature of their crime. Remember though, innocent until proven guilty. Along those lines a few things to consider is that sociopaths and psychopaths come from all walks of life with all types of parents, no matter how good the parent is. Also people with great parents are also brain washed into cults and crimes all the time. Also consider the Newton shooters mom and how much outrage she got, many people said she could and should have done better but no one was calling for her dead corpse to be hung from the rafters. Maybe its because she was already dead, or maybe its because she was white and christian.