According to Sridhara Svami, Arjuna was obliged to catch this son of a brahmana like an animal as a part of his duty (dharma). This suggestion by Sridhara Svami is also confirmed in the later statement of Sri Krsna

Expressions researched:
"According to Śrīdhara Svāmī, Arjuna was obliged to catch this son of a brāhmaṇa like an animal as a part of his duty" |"This suggestion by Śrīdhara Svāmī is also confirmed in the later statement of Śrī Kṛṣṇa"

Srimad-Bhagavatam

SB Canto 1

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.

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.