I'm not defending the guy, but a plausible explanation could be that the ole lady was squeezing him for every nickle the guy had. As I understand it, the woman lived in an affluent neighborhood, and he was a retired sheriff's deputy. With the help of our wonderful family court, this guy was probably paying his entire income to her for alimony and child support. Who knows, maybe she even turned the kids against him.
He was probably looking at another 13+ years (one kid was only 5) of forking all his money over to her, so she could live like a queen. Being already retired, that was pretty near the rest of his life. So rather than live the rest of your life in financial ruin, I can understand the guy wanting to off himself. However, killing your own kid is insane.