the assailant may be armed, may the person under attack in such a case kill him immediately?

The answer is: Yes. If he strongly believes that, were he to defend himself by the least forceful means and then by progressively stronger means, the assailant would kill him, then he may kill him, and there is no blame upon him before Allah.

However, if the heirs of the slain man later claim that he did not attack this man, but that this man was the one who assaulted and killed him and then claimed that he had been an assailant, the killer is told: “Prove that the man attacked you.” He replies: “I can prove it: he attacked me in my house, for I did not kill him in the street; rather, I killed him in my house.” They respond: “Yes, you killed him in your house because you invited him, he accepted the invitation, and you took advantage of the opportunity and killed him.” This is possible—so what is to be done?

The well-known position in the madhhab is that the killer is executed, even though he claims to have been defending himself, unless he establishes that the other man attacked him; in that case, he is not executed.

Shaykh al-Islam Ibn Taymiyyah (may Allah have mercy on him) said that the circumstantial evidence concerning both the killer and the slain person must be considered. If the killer is known for righteousness and uprightness, such that it is inconceivable for him to assault anyone by killing him, while the assailant is known for evil, corruption, and attacking people, then the killer’s statement is accepted with his oath, and he need not produce evidence. What Shaykh al-Islam said is the only view by which people can practically act.

His statement, “the assailant,” includes both human assailants and non-human ones. Thus, if a camel attacks someone and he kills it in repelling its attack, is he liable to compensate the camel’s owner? No, for the camel has forfeited its protected value. And if he kills game that attacks him