Before I had kids I believe a lot in nurture. Now, having two kids, I believe much more in nature. Nurture matters, of course, often in curbing nature... But nature is there first and foremost.
Both my kids are difficult, in their own unique ways. I'm constantly fighting to nurture their difficult natures into good citizenry. My son has been diagnosed with Asperger/Autism, ODD, ADHD, and anxiety. Further testing places him a bit differently, still Aspergers (though incredibly mild on the spectrum), but only ADD, and not ODD. Anxiety is still there. In earlier times he'd not have been diagnosed, and I expect he'd be treated far differently by the schools... If I had parented him differently I suspect I would have had a child suicide on my hands by age 8. It is a constant fight, but I adore him. He's the sweetest, most loving boy ever. But I cringe at what might have been without careful nurture.
My daughter is totally different. My son may not be ODD or ADHD, but my daughter fits the bill far more... She is manipulative, deliberately oppositional and defiant (even as a baby - before nurture could have had any real play in things), goes out of her way to annoy or irritate, lies, and generally is incredibly difficult. She also has a very short attention span, not being able to sit through stories or movies that her peers have no difficulty with. Now, as she's growing older she is greatly improving (thank goodness, I was really starting to despair). But still a real challenge to parent. Thankfully I don't think she has the self-harming potential her brother has/had. But I could easily see her becoming the popular girl in class who tortures the less popular kids in her spare time... I work constantly to help her become more aware of different kinds of people, of how you should treat others, the need for good manners, of why truthfulness is better, etc.
If my kids were raised in a home that didn't believe in really intervening... well I'd really be concerned for their long-term potentials. I'm far from a perfect parent, and am sure I cause as many problems as I solve. But I think my parenting makes a real difference.
So if nature is bad to start with, and nurture doesn't correct it (such as sparks19's neice), I think that some people just have no chance. A perfectly decent parent for the average child could miss the signs of troubles in a child with a bad nature. And even with nurture, I think some kids' problems might be too much to guide to a healthy living.