We have seen that although she was so strict and had her own ways, J and A loved each other. J as a kid might have been influenced by DS teaching but he still stood up for A. Even after he grew older he supported A many times and went against DS's orthodox rules. That meant he knew right from wrong. He even protested his marriage to kid G when that happened. When he was getting attracted to G as an adult too he tried to back off, coz he knew he was wrong. But eventually he gave into 'temptation'. But to satisfy his own ego and coz he did not have the guts to do the right thing he lost his backbone and just wore blinkers to avoid seeing the obvious right thing to do. His ridiculing A was just his way to remove the guilt he had that he was cheating her and his family. It was just a way to make him believe that she is worthless and not ready to change (which he did not even try to do) so that he could be happy with the so called upgrade he was involving himself with.
As a kid sure if a child misbehaves I would hold the parents responsible but after becoming an adult and getting educated every one is solely responsible for their actions. Him constantly trying to blame someone else for his actions is coz he knows very well that he is wrong but is just not man enough to take responsibility for it. He has put himself in a very deep mess and he just doesn't have the maturity nor the courage to know how to come out of it.