We know no such thing. By all accounts, Navalny was fine two days ago. Then he suddenly drops dead while stuck in a prison controlled by the man who has already tried to kill him once - a man who is up for election during the most difficult time in his country's history and who might well want to send a message to anyone daring to oppose him. Furthermore, this man is known to hold grudges and has a history of his enemies dying from falls, poison, Russian suicide (aka gunshot wounds to the back of the head) and suddenly exploding aircraft.
That said, Navalny pissed off a lot of people with his exposes on corruption. There are a number of oligarchs and government figures with ruffled feathers who might want him gone... but the number one suspect has to be Vladimir Putin: motive, means and opportunity all point in his direction.