He should have done more to stand up for his kids and should have been able to realize that his wife was off her rocker, especially after what may have been some sort of psychotic break after people came after the wife for taking away her son's room.

When you give up your sense of self to go after your dream girl, you are all in on that crazy. He did that early on and it became a slippery slope for him. Definitely some serious blame is due but he did not appear to be an active participant in the bad stuff as far as I can tell.