He got stabbed in the back because:
1) He attempted to fight a war on too many fronts simultaneously. The swamp can't be drained overnight.
2) He had an incredibly hard time filling and retaining a cabinet. He repeatedly threw his own appointees under the bus, loudly and publicly, leading many qualified candidates to reject the idea of working in a Trump Admin. This is how we ended up with someone like Barr - he literally had nothing to lose because he was already retired. It's a miracle we got some decent people, like Pompeo, in that climate of fear and mistrust.
3) He appears to only think tactically, not strategically.
4) He massively underestimated the evil of the Democrat Party. He's used to people liking him, and in his heart of hearts he's always thinking, "if I just explain to them my motives, which are altruistic, they will surely realize I am good and will do the right thing." I think he finally figured it out a year ago, but too late.
I think the most costly problem has been underestimating the level and scope of perfidy in not only the bureaucracy, but in the Ivory Towers of the GOP. The swamp is deeper and wider than even he imagined, and many here as well.
The taxpayers are the ones sucking hind teat, and that's why we are. As many here, including myself have repeatedly pointed out, effectively there is no two party system, no real opposition based on principle, and even the justices of the SCOTUS lack the intestinal fortitude and spinal integrity to hold the rest of the government to the Constitution.
This was noted, albeit jokingly (sorta) during the end of the Iraq war, when people asked what sort of a Government they should set up, and were answered with "Let them use our Constitution, we aren't using it."
Well, here we are. An election blatantly stolen, and Americans gutlessly resigning themselves to that because even Americans lack the gumption to protest, despite seeing what sort of street crime the other side has in store. Our ancestors would disown us in shame.
Better get some range time, and sight in those rifles, because if we let this slide when words would do, we'll need them or wear chains.