We don't have any knowledge of his break up and divorce..when it happened, who left who and why, how much the divorce negatively affected his financial position, how the split affected his mental health. His ex wife and her bf appear to be is first target.
Based on his nephew's report about how he screwed him out of his home, I think we can assume that his greed for money defined him. And he didn't go through a divorce without losing money, which probably made his ex-wife his first revenge target.
But if that is even true or relevant, it doesn't explain his continued rampage of setting fires and killing people he knew and people who he didn't know at random. There is an unknown explanation to his story, and I hope that his victim's loved ones we will have answers to "why". They need to know why.