Navalny saw exile as a betrayal of both his country and his ideas and convictions. I think he mentioned that an opposition that is staying outside of Russia would lose moral legitimacy in the eyes of Russians too, or something similar.
I think that unfortunately for him, his support inside Russia was probably never high enough to seriously challenge the existing leadership.
Don't think noble causes. Think money, blackmail. When thinking of timing of his death think of what else was going on in the west at that time. Think Tucker/Putin interview.