Asvatthama was a bona fide son of Dronacarya and Krpi, but because he had degraded himself to a lower status of life, it was proper to treat him as an animal and not as a brahmana

Expressions researched:
"Asvatthama was a bona fide son of Dronacarya and Krpi"

SB Canto 1

Aśvatthāmā was a bona fide son of Droṇācārya and Kṛpī, but because he had degraded himself to a lower status of life, it was proper to treat him as an animal and not as a brāhma.

SB 1.7.33, Translation and Purport:

Arjuna, his eyes blazing in anger like two red balls of copper, dexterously arrested the son of Gautamī and bound him with ropes like an animal.

Aśvatthāmā's mother, Kṛpī, was born in the family of Gautama. The significant point in this śloka is that Aśvatthāmā was caught and bound up with ropes like an animal. According to Śrīdhara Svāmī, Arjuna was obliged to catch this son of a brāhmaṇa like an animal as a part of his duty (dharma). This suggestion by Śrīdhara Svāmī is also confirmed in the later statement of Śrī Kṛṣṇa. Aśvatthāmā was a bona fide son of Droṇācārya and Kṛpī, but because he had degraded himself to a lower status of life, it was proper to treat him as an animal and not as a brāhmaṇa.