Even if it's true that his adoptive parents might have coddled him I don't believe for a second that that alone would create an Russell Greer. It certainly might cause for some people to grow up with bad behavior or traits, but not to that extent.
When it comes to nature vs nurture I believe it'll depend on the individual. Some people might commit certain acts (violence, murder, or less but still problematic behavior) because of abusive parents or fucked up childhood, neglect, etc. But some other people are just born fucked up in head, and in many times genetics is at play in this case. So I think both are real things that can affect someone and both are not an excuse for whatever shit they decide to do.
I 100% believe that Russel fucked up personality is like his face. Not his adoptive parents fault. He was born like that, fucking disgusting, this isn't abusive parents, mommy issues, being coddled as a child, etc. While some coddling or people feeling pity for him looking/acting like a tard might have made things worse, he is what he is by nature. A disgusting, misogynistic, retarded autist.