I have to agree with rabid_squirrel here, one issue that if nurture is bad, that means you had bad chances with nature from the start statistically. Both of my parents suffer from mental illness, so I suffer from a mental illness, and (with many possible exceptions, I was unlucky here) their mental illnesses had an adverse effect on my nurture. I was all but neglected by my parents (In favour of making sure my sibling was taken care of) until later it turned into emotional & verbal abuse very rarely crossing the line to physical, and I became the family scapegoat
That said, during all this my sibling was welcome to do whatever they liked to me, which usually consisted of violent acts & getting me punished for things I hadn't done. Not sure if that would fit your parameters.
Anyway, my thoughts are that both have an effect, and feed into eachother, but one or the other can be enough on it's own to be too much. It's all subjective and a matter of "luck" with genetics & situations