I'm not a psychiatrist.
That said, I suspect that McCain is also settling a score with US, the voters.
After all, we chose to not elect him, and did elect Trump.
He has no idea how, in the twilight of his career, just how much better he would look if he sided with our very fine country instead of a descent into petty envy and child-like jealousy.
.
He’s a spoiled entitled child his life!