Though man is essentially divine in nature, he is prone to vice and committing crimes, thus incurring pātaka or pāpa or sin. This is mainly due to the evil tendencies carried over from the past lives.
Etymologically, the word pāpa (sin) is defined as ‘that from which one has to protect oneself’. It arises first in the mind as the tendency towards vice and manifests itself either in neglecting the prescribed duties or in committing prohibited actions. The scriptures are the primary, if not the sole, authority in this field.
The dharmaśāstras—the works that deal with the religio-social life of the Hindus—have classified sins into two major groups: the mahāpātakas or atipātakas (major or mortal sins) and upapātakas (minor or venial sins).
Brahmahatyā or the killing of a saintly brāhmaṇa, is one of the mahāpātakas that is often mentioned in these works. Any act that will lead either indirectly or directly to the slaying of a brāhmaṇa is brahmahatyā. Even incitement or approval is liable to be branded as brahmahatyā. And it entails unbearable suffering.
One does not incur brahmahatyā if one kills a brāhmaṇa who is an ātatāyin (a criminal guilty of grave offences), in self-defence.
With regard to its expiation, there does not seem to be any unanimity among the writers of these works. It ranges from death, up to undergoing severe austerities like offering one’s limbs into fire, walking very long distances with very little or no food, performing sacrifices like Aśvamedha or Abhijit or Gosava and so on.