ŚB 1.7.39

The son of Droṇācārya is condemned here as the burnt remnants of his family. The good name of Droṇācārya was very much respected. Although he joined the enemy camp, the Pāṇḍavas held him always in respect, and Arjuna saluted him before beginning the fight. There was nothing wrong in that way. But the son of Droṇācārya degraded himself by committing acts which are never done by the dvijas, or the twice-born higher castes. Aśvatthāmā, the son of Droṇācārya, committed murder by killing the five sleeping sons of Draupadī, by which he dissatisfied his master Duryodhana, who never approved of the heinous act of killing the five sleeping sons of the Pāṇḍavas. This means that Aśvatthāmā became an assaulter of Arjuna’s own family members, and thus he was liable to be punished by him. In the śāstras, he who attacks without notice or kills from behind or sets fire to another’s house or kidnaps one’s wife is condemned to death. Kṛṣṇa reminded Arjuna of these facts so that he might take notice of them and do the needful.