Personally I think it was because the power got to his head and he thought that he was the most powerful wizard and no one could defeat him anyway, and then the shame of it he was defeated by a young adult. Fellow potter fan here good question.
Personally I think it was because the power got to his head and he thought that he was the most powerful wizard and no one could defeat him anyway, and then the shame of it he was defeated by a young adult. Fellow potter fan here good question.