PURPORT

An enemy, who sets fire in the house, administers poison for eating, attacks all of a sudden with sharpened weapons, and plunders away wealth or usurps agricultural fields and entices one's wife is called an aggressor. Such aggressor though he may be a brahmin or the so called son of a Brahmin, has to be punished in all circumstances. When Arjuna promised to behead such aggressor namely Aswatthama he knew it well that Aswatthama was a son of Brahmin but because the so-called Brahmin acted like a butcher, he was taken as such and there was no question of sin for killing such a Brahmin's son who proved to be a villain.