mainaṁ pārthārhasi trātuṁ brahma-bandhum imaṁ jahi yo 'sāvanāgasaḥ suptān avadhīn niśi bālakān
Translation
Lord Sri Krishna said, "Oh Arjuna you ought not to show any mercy for his releasing this relative of a Brahmin who has killed innocent and sleeping boys without any fault."
Purport
The Brahmabandhu is significant. A person who happens to take birth in the family of a Brahman but is not qualified enough to be called a Brahmin is addressed as the relative of a Brahmin and not as a Brahmin. The son of a High court judge is not virtually a high court judge but there is no harm to address a high court judge's son, as a relative of the Hon. Justice. Therefore, as by birth only one does not become a high court judge, so also one does not become a brahmin simply by birth right but by acquiring the necessary qualification of a Brahmin. As the high court judgeship is a post for the qualified man so also the post of a Brahmin is attaindable by qualification only. The Shastra enjoins that even if such good qualifications are seen in a person born in the family of other than a Brahmin, the qualified man has to be accepted as a Brahmin and similarly if a person born in the family of a Brahmin, is void of Brahminical qualification, must be treated as a non-brahmin or in better terms a relative of a Brahmin ! Lord Sri Krishna the Supreme Authority of all religious principles the Vedas, has personally pointed out this differences and he is about to explain the reason for this in the following slokas.