Provocation is no ground for exempting one from criminal liability for one’s act, but only maybe ground for mitigating the punishment.
At common law, provocation will sustain only if met four tests:
a. the provocation must have been one that would arouse sudden and intense passion in the mind of an ordinary person such as to cause him to lose his self-control;
b. the defendant must have in face been provoked;
c. there must have been no sufficient time between the provocation and the killing for the passions of a reasonable person to cool; and
d. the defendant in fact did not cool off between the provocation and the killing.
Usually, the rule would be applied narrowly onto limited scenarios, such as " a personally serious battery," “a threat of deadly force," or “discovering one’s spouse in bed with another person."
In this case, the taxi driver may claim provocation for that he had been surrounded by angry riots with possible threats of deadly force. This was why I said that the taxi driver may be incriminated as murder or voluntary manslaughter.
People could claim self-defense on the behavior of dragging the driver out of the cab. However, these attackers can never successfully claim that they were defending themselves by using such brutal violence on the driver in a coma. Of course, the “provocation rule" does not apply here. Since once the driver had been out of the cab, there was no provocation at all. And the violence they used exceeded the necessary standard of self-defense.
關於普通法的義憤殺人（on voluntary manslaughter at common law）