Srimad-Bhagavatam: Canto 1 - Chapter 1 - Verse 53-54


श्रीभगवानुवाचब्रह्मबन्धुर्न हन्तव्य आततायी वधार्हण: ।मयैवोभयमाम्नातं परिपाह्यनुशासनम् ॥ ५३ ॥कुरु प्रतिश्रुतं सत्यं यत्तत्सान्‍त्वयता प्रियाम् ।प्रियं च भीमसेनस्य पाञ्चाल्या मह्यमेव च ॥ ५४ ॥


śrī-bhagavān uvācabrahma-bandhur na hantavyaātatāyī vadhārhaṇaḥmayaivobhayam āmnātaṁparipāhy anuśāsanam


The Personality of Godhead, Śrī Kṛṣṇa said: A friend of a brāhmaṇa is not to be killed, but if he is an aggressor he must be killed. All these rulings are in the scriptures, and you should act accordingly. You have to fulfill your promise to your wife, and you must also act to the satisfaction of Bhīmasena and Me.


Arjuna was perplexed because Aśvatthāmā was to be killed as well as spared according to different scriptures cited by different persons. As a brahma-bandhu, or a worthless son of a brāhmaṇa, Aśvatthāmā was not to be killed, but he was at the same time an aggressor also. And according to the rulings of Manu, an aggressor, even though he be a brāhmaṇa (and what to speak of an unworthy son of a brāhmaṇa), is to be killed. Droṇācārya was certainly a brāhmaṇa in the true sense of the term, but because he stood in the battlefield he was killed. But although Aśvatthāmā was an aggressor, he stood without any fighting weapons. The ruling is that an aggressor, when he is without weapon or chariot, cannot be killed. All these were certainly perplexities. Besides that, Arjuna had to keep the promise he had made before Draupadī just to pacify her. And he also had to satisfy both Bhīma and Kṛṣṇa, who advised killing him. This dilemma was present before Arjuna, and the solution was awarded by Kṛṣṇa.