I agree that financial trouble plays a major role in the killings. I don't think medical expenses played a big role (though maybe). Instead, I think they spent more than they earned and they didn't pay their bills. Simple as that.
All the other stuff is bad, too, of course. He might have strayed because of the financial situation as well - a way of running away rather than rolling up his sleeves and doing what it takes to solve the money issues, like downsizing, getting a second job, etc. People can survive financial crises - he chose to destroy his family instead.
I do think perhaps the pregnancy ruined his story that he and his wife were no longer intimate. With SW's public SM, it wouldn't be hard for APs to find out the truth. And, if he was fearful of same-sex relationships being exposed, that is likely a big factor too.
But, I think the crux of all this is money problems.
jmo