Ukraine

Trying to get into a Russian-like mindset (impossible, really, despite my past experience with them), maybe it's just a real life version of the new-in-prison or the Pride-Male trope. He had to be seen to take on the strongest comer to resolidify his power.

Sorry, but I'm gonna stereotype here: Russians just really suck as a collective. We're all ugly, selfish humans, and we're mostly worse in our tribal form, but the Russians are really bottom of the barrel.
I don't think its anything uniquely Russian really, except maybe in a modern context. Read about some of the shenanigans that went on with Hitler and Napoleon, or really any autocrat/dictator and this really isn't that unusual. You can go from favored son to sheetlist pretty quickly.

In a US context, its comparable to all the back door politicking and backstabbing that went on in the American Civil War, particularly early on in the Union side. But because the US ostensibly has things like "rule of law", "civillian control of the military", etc. when these types of things would go down, people would simply be "reassigned" (or retired)