[robbiewxyz]

I've wondered the same. A look at his contemporary media quotes gives maybe a hint. From Navalny: "It is difficult for me to understand exactly what is going on in [Putin's] mind. [...] 20 years of power would spoil anyone and make them crazy. He thinks he can do whatever he wants."

If this quote is genuine, as opposed to wistful, it suggests Navalny's evaluation of Russia was that Putin couldn't, in fact, do whatever he wanted there. As best I can tell, such an evaluation would have been pretty damn close to completely inaccurate.

The choice to return to Russia as a catastrophically-failed gamble based on that premise is what makes the most sense to me.