SB 1.7.35

mainaṁ pārthārhasi trātuṁ
 brahma-bandhum imaṁ jahi
yo ’sāv anāgasaḥ suptān
 avadhīn niśi bālakān
Synonyms: 
enam — never unto him; pārtha — O Arjuna; arhasi — ought to; trātum — give release; brahma-bandhum — a relative of a brāhmaṇa; imam — him; jahi — kill; yaḥ — he (who has); asau — those; anāgasaḥ — faultless; suptān — while sleeping; avadhīt — killed; niśi — at night; bālakān — the boys.
Translation: 
Lord Śrī Kṛṣṇa said: O Arjuna, you should not show mercy by releasing this relative of a brāhmaṇa [brahma-bandhu], for he has killed innocent boys in their sleep.
Purport: 

The word brahma-bandhu is significant. A person who happens to take birth in the family of a brāhmaṇa but is not qualified to be called a brāhmaṇa is addressed as the relative of a brāhmaṇa, and not as a brāhmaṇa. The son of a high court judge is not virtually a high court judge, but there is no harm in addressing a high court judge’s son as a relative of the honorable justice. Therefore, as by birth only one does not become a high court judge, so also one does not become a brāhmaṇa simply by birthright but by acquiring the necessary qualifications of a brāhmaṇa. As the high court judgeship is a post for the qualified man, so also the post of a brāhmaṇa is attainable by qualification only. The śāstra enjoins that if good qualifications are seen in a person born in a family other than that of a brāhmaṇa, the qualified man has to be accepted as a brāhmaṇa, and similarly if a person born in the family of a brāhmaṇa is void of brahminical qualification, then he must be treated as a non-brāhmaṇa or, in better terms, a relative of a brāhmaṇa. Lord Śrī Kṛṣṇa, the supreme authority of all religious principles, the Vedas, has personally pointed out these differences, and He is about to explain the reason for this in the following ślokas.