SB 1.7.16
tada sucas te pramrjami bhadre
yad brahma-bandhoh sira atatayinah
gandiva-muktair visikhair upahare
tvakramya yat snasyasi dagdha-putra
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
O gentle lady, when I present you with the head of that brahmana, after beheading him with arrows from my Gandiva bow, I shall then wipe the tears from your eyes and pacify you. Then, after burning your sons’ bodies, you can take your bath standing on his head.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
An enemy who sets fire to the house, administers poison, attacks all of a sudden with deadly weapons, plunders wealth or usurps agricultural fields, or entices one’s wife is called an aggressor. Such an aggressor, though he be a brahmana or a so-called son of a brahmana, has to be punished in all circumstances. When Arjuna promised to behead the aggressor named Asvatthama, he knew well that Asvatthama was the son of a brahmana, but because the so-called brahmana acted like a butcher, he was taken as such, and there was no question of sin in killing such a brahmana’s son who proved to be a villain.