Z endeavours to  strike A with a horsewhip, not in a way that would cause severe harm to
A. In response, A retrieves a pistol. Despite this, Z continues the assault. A, having a genuine belief that there is no other way to prevent being horsewhipped, shoots and kills Z. What offence has A committed?

Culpable homicide not amounting to murder , Explanation: IPC Section 299. Culpable homicide.—Whoever causes death by doing an act with the intention of causing death, or with the intention of causing such bodily injury as is likely to cause death, or with the knowledge that he is likely by such act to cause death, commits the offence of culpable homicide.Section 300. Murder.—Except in the cases hereinafter excepted, culpable homicide is murder, if the act by which the death is caused is done with the intention of causing death, or—  2ndly.—If it is done with the intention of causing such bodily injury as the offender knows to be likely to cause the death of the person to whom the harm is caused, or—  3rdly.—If it is done with the intention of causing bodily injury to any person and the bodily injury intended to be inflicted is sufficient in the ordinary course of nature to cause death, or— 4thly.—If the person committing the act knows that it is so imminently dangerous that it must, in all probability, cause death, or such bodily injury as is likely to cause death, and commits such Section 304. Causing Death by Rash and Negligent Act- act without any excuse for incurring the risk of causing death or such injury as aforesaid. Causing death by a rash and negligent act involves causing someone's death due to negligence. The negligence must be so gross as to indicate a disregard for human life.It is punishable with imprisonment for a term which may extend to two years, or with a fine, or with both.