Thus, in reality, he has brought about his own death, so the one who killed him bears no liability. However, he must repel him by the least harmful means possible. If a threat is sufficient to repel him, he should not strike him; if a light blow is sufficient, he should not strike him severely; if a severe blow is sufficient, he should not kill him; but if he cannot be repelled except by killing him, then he may kill him.
If someone were to say: It has been reported that if a man finds someone with his wife and kills him, there is no liability upon him (1), even though the person could have been repelled by means short of killing. Yet you say that an assailant may not be killed if he can be repelled without killing.
The answer is that killing a person whom one finds with his wife or a woman within his unmarriageable kin is not a matter of repelling an assailant; rather, it falls under punishing the aggressor. Since the aggression has already occurred, he is killed as a punishment, not in order to repel his aggression. Thus, there is a difference between the two cases. A comparable example is that the Prophet, peace and blessings be upon him, waived liability for the eye of one who looks through a crack in the door, should a person put out his eye without first warning him (2). This too falls under punishing the aggressor, because the aggression has already taken place; it is not a case of repelling an assailant. Repelling applies to something that has not yet occurred, whereas once something has occurred, there remains only punishment.
So if the person being assaulted does not know whether he may immediately kill him, because