Asvatthama was the representative of Dronacarya, and therefore killing Asvatthama would be like killing Dronacarya. That was the argument of Draupadi against the killing of Asvatthama
SB Canto 1
He (Droṇācārya) is certainly still existing, being represented by his son. His wife Kṛpī did not undergo a satī with him because she had a son.
The wife of Droṇācārya, Kṛpī, is the sister of Kṛpācārya. A devoted wife, who is according to revealed scripture the better half of her husband, is justified in embracing voluntary death along with her husband if she is without issue. But in the case of the wife of Droṇācārya, she did not undergo such a trial because she had her son, the representative of her husband. A widow is a widow only in name if there is a son of her husband existing. So in either case Aśvatthāmā was the representative of Droṇācārya, and therefore killing Aśvatthāmā would be like killing Droṇācārya. That was the argument of Draupadī against the killing of Aśvatthāmā.