By firing McCabe, Trump has significantly bolstered his attorneys’ ability to credibly argue that any testimony McCabe might give which is harmful to Trump’s interests is motivated primarily by McCabe’s desire to get revenge on Trump for stripping away his pension – and therefore that McCabe’s testimony should be disregarded. I doubt that is a coincidence.
Never mind that Trump is guilty of “getting back” at those he perceives as not in his very small and protected corner.
It also sends a message to others in the FBI/DOJ — about things like personal vulnerability if you press boundaries under the wrong circumstances, no matter how high up the ladder you are.