They tricked him by saying his son Aswasthama is dead but Aswasthama is an Astachiranjibi he never dies. Drona did not believe so asked Yudhistir. Bheem had killed an elephant named Aswasthama, so Yudhistir said "Aswasthama hato hata" something like Aswasthama is dead and quietly "elephant" but Drona did not hear that. So he sat down on the chariot, dropped his weapons and closed his eyes. Dristashyumna(Draupadi's brother) seeing this cut Drona's head thereby taking revenge for his father's death. This was clearly a cowardly act. There are a lot of cowardly acts in Mahabharat even by good guys.