It's a combination of both. Absolutely no way can it just be one or ther other. My teacher drilled into our heads that it's "nature THROUGH nurture".
There are several theories about how we develop but we'll start with Freud's Psychosexual theory of development and I'll use Casey Anthony as an example.
When a child is born he starts in the "oral" stage. He's a helpless, defenseless, reflexive creature. This means for about the first 3(?) months of his life, everything he does is simply an involuntary reflex. The main reflex infants have is crying and sucking. This is the only way they send/recieve information from the outside world. When the baby get's upset he reflexively cries and his mom MUST come to him every single time and makes the things that upset him go away. With consistensy, the baby learns that touch=comfort and mom=trust. But if mom does NOT consistently come when her baby cries, the child does not have a predictable environment and he cannot learn comfort or trust. Eventually the baby will simply not cry when he is upset, because why cry if nobody will come help you? Have you ever seen a baby get bumped in the head or fall and wasn't shaken in the least? Why cry if nobody is going to help you? My mother did this when I was an infant, and it's part of the reason why I have such trouble asking for help. Sometimes I feel extremely frustrated and overwhelmed and I don't know what to do with those emotions so I cry. This is where that comes from. People who tend to suck their thumbs, smoke cigarettes, become alcoholics, pop pills, what do these things have in common? They all satisfy a need for oral stimuls. They get great comfort from sucking on moms boob!
Take Casey Anthony. She is a classic example of an orally fixated person. See if you can find a pattern here. She told lies about her family, her job, her parents (oral). Her parents also told lies (oral). She gave her baby Xanax pills (oral) to keep her from crying while she went out and partied. When the baby died she buried it with a heart sticker over its mouth (oral) and wrapped its head in a blanket.
If he doesn't learn to trust his environment, he cannot move on to the next stage of development. He will not have the initiative to move on from simply observing. Around age 1-3 babies start experimenting with urine and feces. This is the Anal stage. When we potty train kids we teach them to surpress natural bodily functions. Sometimes if you potty train too early or are too harsh about it, it can cause obsessive need for control, order, and uptightness. Ever hear some say "Dude quit being so anal!" He's anal retentive, constipated really! Next time you see someone being anal, tell them to go take a shit. Let go of all that tension.
These are just two stages of Freuds theory, and I can't remember the other guy's theory but he kinda picks up where freud leaves off. ANYWAY, there's a ton of factors to consider and this is just one miniscule part of it. It is aboslutely 100% the parents responsibility for the way their child turns out. Absolutely. But of course there are certain aspects of genetics we can never escape. A predisposition for mental illness is an obvious one. Believe it or not there are psychopaths who do not turn out to be murderers or scandals. A psychologist interviewing infamous serial killer "The Iceman" put it really well. Basically you have children born without fear or slight lack of emotion. Now you can raise them with trust and comfort and they become people like firefighters, Sky Divers, ultimate thrill seekers. Or you can raise the child in an unpredictable environment where everything is a threat, and he only learns to hurt. He becomes the crooked politcian, axe murderer, rapist.
Sorry this was kind of all over the place. It's hard to summarize this kind of concept because it's really complex. This is a really good book about Tommy Lynn Sells a serial killer who got away with dozens of murders.