SB 1.7.39
tad asau vadhyatam papa
atatayy atma-bandhu-ha
bhartus ca vipriyam vira
krtavan kula-pamsanah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
This man is an assassin and murderer of your own family members. Not only that, but he has also dissatisfied his master. He is but the burnt remnants of his family. Kill him immediately.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The son of Dronacarya is condemned here as the burnt remnants of his family. The good name of Dronacarya was very much respected. Although he joined the enemy camp, the Pandavas held him always in respect, and Arjuna saluted him before beginning the fight. There was nothing wrong in that way. But the son of Dronacarya degraded himself by committing acts which are never done by the dvijas, or the twice-born higher castes. Asvatthama, the son of Dronacarya, committed murder by killing the five sleeping sons of Draupadi, by which he dissatisfied his master Duryodhana, who never approved of the heinous act of killing the five sleeping sons of the Pandavas. This means that Asvatthama became an assaulter of Arjuna’s own family members, and thus he was liable to be punished by him. In the sastras, 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. Krsna reminded Arjuna of these facts so that he might take notice of them and do the needful.