The divorce was all her idea. He was blindsided by her request, and did not want it. According to his attorney, Peter was distraught over it. He gave her MORE in support than required by law, just to be sure they were well taken care of, $240,000 per year. She also stipulated in the divorce that she was to have sole responsibility for making decisions (health, etc) for Adam. He had no say.
Perhaps Adam was angry at his mom for pushing his father out of his life.
Your information definitely paints a different picture. I have looked on the web for that information about Peter Lanza and have run through the same repetitive story; much of which is cut and pasted from each other.