One thing I posted earlier looking at his gait and body posture on the video was that he walked like an ex-atlete. He did play football, it seems, and well. And seemed to be a good student. So if he was popular in school because of his jock/preppy abilities, he might have flied under the radars, and noticeable problems surfaced later in life.
I think that we'll never understand "why?" since we are not prone to solving our personal problems by murder.
Probably the word "mine" means something different for McKee than for the most of us. Maybe he had to beg Monique to marry him, and in Spencer, she found a man who she was truly happy to be with. I thought that McKee did not have a good concept of boundaries and understanding that another human has the right for autonomy and respect, in partnership.
If Monique was his only wife, then he likely has some traits that make him "impossible to live with". If there was the second wife who also lasted a few months, then, doubly so. We all thought, OK, wife has left, go through therapy, finish residency, start dating again. It happens to many. But doctor/no doctor, maybe he was simply unwilling to spend money on dates and then came to the conclusion that "all women are after money", or such.
I suspect that two things that have contributed were: the forthcoming five-year-anniversary of Monique+Spencer's marriage and the end of the year. And, him being 39, one more year has passed, "what have I achieved", etc.
But ultimately, we'll never understand, because one's mind usually thinks along the lines of "minimizing the damage", not "maximizing" it.