Napoleon took Moscow with the belief that it would lead to a quick conclusion of the war. The problem was that taking Moscow just made the Russians more angry, and, in taking Moscow, he failed in one of his other main goals, that being to destroy the Russian army. The battle at Borodino was fierce and lead to many deaths on both sides, but it was a strategic victory for the Russians in that they could reinforce while the French lost 27,000 men they couldn't replace.

Napoleon's mistake, if any, was not retreating when he failed to take the Russian army and lost 5/6 of his army just getting to Moscow. His second mistake was not retreating when the Russians failed to surrender after month.

Well they burned the city to the ground. He didn't expect this nor could even believe it All the supplies he was hoping to take from the city were gone. His supply trains were under assault. He couldn't supply his troops.

He also thought that the taking of Moscow would be the end of the war.