Kṣatriya—how to become strong, stout, brave, no going away when there is challenge, not to go away from fighting, to possess land, to rule over, īśvara-bhāvaś ca (BG 18.43), and charity. These are the kṣatriya qualifications. The charity was given by the kṣatriyas. Even there are instances that Muhammadan rulers in this country, they also gave in charity land and temple in Vṛndāvana. There are many instances. Aurangzeb gave some land, Jahangir gave some land. There is still one temple, it was constructed by Jahangir, and the other side of the Yamunā there is a village called Jahangir-pura. That village was given to the brāhmaṇas for maintaining the temple. So charity, that is kṣatriya's business, and perform yajñas, give in charity, to rule, not to go away from fighting, challenge, very strong, stout—these are kṣatriya qualification.
And the vaiśya qualification—agriculture. Kṛṣi. Kṛṣi-gorakṣya, and cow protection. Kṛṣi-gorakṣya-vāṇijyam (BG 18.44). And if there is excess, then vāṇijya, trade. Otherwise there is no question of trade. And vaiśya . . . And śūdra, paricaryātmakam—to work for some payment. That is this blacksmith, goldsmith, weaver. You take some work from him and pay him something, maintain him. That is śūdra. So in the śāstra it is said, kalau śūdra-sambhavaḥ. In the Kali-yuga almost everyone is a śūdra. You'll find they're interested in accepting some service. Even one is born of a brāhmaṇa family, he is looking after some good job, that is śūdra mentality. That is not brāhmaṇa's business. Brāhmaṇa will not accept anyone's service, neither the kṣatriyas, neither the vaiśyas. Only śūdras.
So considering all these points, Aśvatthāmā is accepted here as brahma-bandhu. And at the same time he's aggressor. Brahma-bandhoḥ ātatāyinaḥ. Therefore Arjuna promised, "I shall kill him." Arjuna did not promise to kill a brāhmaṇa. No. That was his not business. Because he's proved to be brahma-bandhu and ātatāyinaḥ, he deserved to be killed. Therefore he decided.