SB 1.7 - Arjuna's dilemma
Kṛṣṇa smilingly told Arjuna that a friend of a brāhmaṇa was not to be killed, but if he is an aggressor, he had to be killed. So Arjuna had to fulfill his promise to his wife and also at the same time act for the satisfaction of Bhīma and the Lord. Droṇa was a true brāhmaṇa but because he stood in the battlefield he was killed. Aśvatthāmā was an aggressor but he stood without any weapons. Thus he cannot be killed. And Arjuna had promised Draupadī that he would kill his sons’ murderer. He had to please Bhīma who wanted Aśvatthāma to be killed; he had to please Draupadī who did not want him killed; and he had to please the Lord because both Bhīma and Draupadī were dear to Him. This was the dilemma before Arjuna.