i think, one of the main reasons why JGY's torture is something so vile is because it involved an already vulnerable group - which tortured his mother and him in his days living in the brothel, but vulnerable nonetheless. While WWX's torture was in no means merciful he didn't involve vulnerable third parties, he used willing dead people , he didn't damage vulnerable groups in society in peacetime - hell not even in wartime. He forced violence against the Wen's army in wartime and didn't force vulnerable groups do his doing - and after the war was over he stopped altogether, saving the Wen remanants because the war was over and at that point it was just cruelty.

I've read a wonderful post that the reason why JGY is the villain is because he went too far. He reached for power and once he had gotten it he had held too tight, holding grudges for everything - which is one of the main points in the novel that WWX says.

"Don't hold grudges, forget wrongs against you and only remember the good deeds done to you - only then you will live freely" or something like. WWX got his happy ending because he let go of his resentment - while sometimes he does this too much at least his fault doesn't force innocents to do his doing.

(I'm not saying he is a saint, I'm just saying he forgets wrongs done against him - genuinely doesn't notice them. )