It isn’t about the spouse; it is about the children. If a divorced guy finds himself out of the home, having some other guy living with his kids while he pays for it, why would anyone expect a different outcome? Not defending it, but explaining it; I couldn’t imagine living through a scenario like that.
The media is doing a good job of raking her over the coals (she entered a drug treatment program) but not such a good job of dissecting him. She did have a long standing steady job.
He was in a psychiatric program in Doylestown, taking meds, claimed PTSD, spent 2.5 months deployed (claimed asthma to get sent home), had 3 DUI's over 10 years, one friend said he liked "being" a marine more than he liked "working" as a marine. But did the guy just hatch somewhere? Nothing about even where he is from, where he went to school, etc. Does he have family? He had no job that was mentioned.
He took his new wife to the children's activities when their mother was there. She would have more motive to shoot him, given your explanation.