I think in MDZS there is a fair share of nurture as well as nature. if Xue Yang lived similarly to WWX and didn't lose a finger he would still kill dogs, hunt and hound them and deeply resent and wish to take revenge on/or slaughter the Jiangs; that is his nature to think of himself as more important than the others; the nurture part comes in with the fact that if he had people like Xiao Xingchen earlier in his life; just like he stopped "night hunting walking corpses" he wouldn't have acted out on those impulses; however you cannot deny that being favoured by the jins, he did have stability; he chose to abuse it. he put xiao xingchen giving him chocolates and showing him kindness as of more stable of a state (which is right) yet the moment he is pushed up in a corner, he reverts back to his NATURE. and in his nature, he hurts others to protect himself. if others did x to him he can kill not only the person who did it but also everybody else who is close to that person, who didn't deserve to die. yet, he won't see HIS actions as crimes. won't regret them. thus, Wei Wuxian says, "if you had to kill someone, you should've executed yourself" because some characters have a strong moral code and fail to adhere (nie mingjue, lan xichen), some's adherence to their moral code makes them fail (wei wuxian, xiao xingchen), some do not have a moral code (jin guangsham), some are willing to push it aside for revenge (nie huaisang) and some twist around their watching frames to justify their immoral behavior (Jiang Cheng, xue yang, Jin Guangyao)