The murder (hatya) of a Brahmana, the highest social scale, today called caste. One became a Brahmin through wisdom, not – as today – through birth.
Killing a Brahmana was the worst offense in Vedic times.
Brahmanahatya pervades many myths, it either had to be atoned for during lifetime or it led to unpleasant rebirth.
Shiva once cut off the fifth head of Brahma (the Creator God, a Brahmin) and was thus guilty of this offense. See Shatarupa.