Dronacharya was killed by Drishtadyumna, the brother of Draupadi and prince of Panchala. This was part of a planned strategy - Drona was demoralized by the false news of Ashwathama's death, and Drishtadyumna seized the opportunity to attack him. Arjuna, Yudhishthira, and Satyaki were not Drona's killer.