I get where you are coming from in that it does not make sense that he murdered them. It also makes no sense the amount of drugs he self reported taking. I have no doubt he is a major dealer and laundering all that money.
That said, I do think he murdered them because he actions were getting more and more unpredictable. I think the annihilator in him could not stand that MM and PM were questioning him.
I found this article about another case but the description seems to fit.
“Roughly a quarter to a third of family annihilation cases appear to have financial problems at their root,” Websdale said. “Often what we see here is a deep sense of male shame.”
Perpetrators of familicide are often “highly repressed individuals,” Websdale said, facing potential eviction or bankruptcy. The family may be facing destitution, but “appear on the surface to be respectable.”
The goal, Berrill said, is to spare the family “all these assaults to their sense of well being and their sense of normalcy. It reflects very distorted thinking and very strained and depressive type thinking and desperation which culminates in this kind of terrible act.”
This act is called “anomic familicide,” according to Dr. Kevin Barnes-Ceeney, assistant professor of criminal justice at the University of New Haven. “It’s when you feel like you're going to lose everything and you have an attempt to immortalize the family.”
Family annihilators: The psychology behind familicide