the supply line thing did him in just as much as winter.
Napoleon kept playing into the hands of the Russians and went belly up as a result. He expected the Russians to come out and fight, as they usually did. They didn't. they retreated, and Nap foolishly followed, against the advice of his staff. His staff offered alternative attack routes which would allow for greater flexibility, but he didn't listen. he just kept going straight in, hoping and praying that one more victory would bring the Russians to the table. The Russians were under immense pressure domestically because they kept retreating. Nap eased all that pressure by doing what the Russians wanted and making the retreat/scorched earth policy work. Domestically, it would have been a nightmare for the Russians if Nap had laid back and not pursued.
Nap made a lot of mistakes. Declaring against Russia was not as big a blunder as how he chose to prosecute the war. (edit). war with Russia was almost inevitable (no war is inevitable, imo). So, it's not a question of if, but a question of how.
IMO, one of his biggest blunders was the continental system. He didn't have the kind of power to make it work for the length of time he needed it too. It was the root of the Russian war, and ended up hurting the French worse than it helped.