Why Are XHusbands Such Dicks?

About a month ago, M's dad called me at work and started yelling at me. Yelling at me about every financial decision I had ever made during our life together. Everything from the cars I bought, the student loans I took out, to the computer I bought and how much he pays me in child support for M. All I could do is sit at my desk and cry as quietly as possible. I couldn't yell back because I was work. I couldn't tell him that he needed to open his eyes and look at the amount of money he'd spent on pot over the last decade. The conversation ended with him threatening to take me to court because he didn't feel the current child support payment was fair. Really, I think he's just short the money he needs to go hunting. What would he do with extra money every month anyway? More pot, more alcohol, more vicodin, and more hunting.
Today I gave M's dad five weeks notice that I would be traveling for work for a week (as compared to the usual 3 days notice he gives me when he changes week nights on me). The response I got was somewhat less than stellar, "That really fucks things up for me. I had plans that week." Maybe I should quit my job. I wonder how much he'd like that child support payment.
After this conversation, I get an email from him that says "Child support - you have a chance to treat me fair". Child support is calculated by the state. I did not pull a number out of thin air or decide to arbitrarily screw him for everything I could take. Let's talk about fair. Where exactly was it fair that he could spend money on drugs while we were married and take money away from our family's goals? Where was it fair that he could spend most of his time stoned and checked out of life? Where was it fair he could withdraw from any and all activities that didn't involve his friends? He says he would try to get me back if he thought he had a chance. He's already lost his wife and he still doesn't see the impact of his behavior. I don't think he'll ever grow up.