Its gotta be nurture. People aren't born like that. They are taught (nurtured)
I think definately nurture too.
Parents play a huge role but there're other factors that create the person too, like how they get on with others (social skills), the different experiences and the persons perception of them, the perceptions they create of themselves, what they're exposed to, etc, etc. You could go on and on with the many reasons.
Pushka, mostly nice parents raise nice kids but sometimes they don't teach the necessary skills that help children steer their way successfully through life. I think that's when other negative factors come into play.
You've got to have all the bad ingredients to be a disaster and the right ones for success. Most people are somewhere in between.
Funnily I've only just finished reading 'Born or Bred ? - Martyn Bryant: the making of a mass murderer.