I do not think he planned this his whole life, nor did he lie about loving his wife and children. I think he started out a happy family man. He didn't lay in wait for this day to murder them all.
I think it is something that slowly built up inside him. He had never had a serious relationship before. This was all new to him. This was his first real girlfriend, and before he knew it, he had 2 little daughters, a big house with big house payments, a pregnant wife, with her own medical issues, lots of medical bills from his family, and a past bankruptcy, with a possible new one looming. He worked hard at a job that was not all that exciting.
He began to greatly dislike his current life. His wife used to cater to him, dote on him, when they were newly in love. But now she had the kids and 'her people' and her sales career and her social media life. And now the pregnancy...was their 3rd child going to have medical issues too?
CW was a quiet, simple man and his life had become chaotic and complicated. His wife liked the finest things in life and he worked hard to try and keep up with their expenses. He wanted to provide for his family so he spared no expense. But soon, he was barely treading water, once again. And there was no end in sight, with the upcoming birth and the new mouth to feed.
His girl friend understood him. She catered to him and gave him a nice quiet place to relax. It became harder and harder to imagine sticking it out in his family life.
He never showed anger or frustration. He just stuffed it and stuffed it...until he couldn't do so anymore...