A attacks Z under such circumstances of grave provocation that his killing of Z would be only culpable homicide not amounting to murder. B, having ill-will towards Z and intending to kill him, and not having been subject to the provocation, assists A in killing Z. Here

This scenario is a direct illustration of Section 38 of the Indian Penal Code (IPC) (corresponding to Section 10 of the Bharatiya Nyaya Sanhita). The legal principle established here is that multiple persons engaged in a single criminal act can be guilty of different offences based on their individual intentions and the circumstances surrounding their actions.

• A's Liability: Because A acted under "grave and sudden provocation," his act is mitigated under the exceptions to murder. Thus, A is guilty of culpable homicide not amounting to murder.
• B's Liability: B acted with a pre-existing intent to kill (malice aforethought) and was not influenced by the provocation. Therefore, B's act meets the criteria for murder.

Joint participation in a physical act does not automatically result in identical criminal liability if the mens rea (guilty mind) differs between the participants.