Navalny was very clear why he went back. Here's a reading of one of his final letters, which I highly recommend everyone watch https://www.youtube.com/watch?v=25qEVGqPwLk
Why return only after being poisoned and not earlier?
> Russian politics over the last decades has so inculcated society with cynicism and conspiracy theory that people fundamentally do not believe in simple motives.
He was poisoned inside Russia in late 2020, and was extracted (by his family, he was in a coma) to Germany for treatment. He returned immediately after recovering in early 2021.