What is the ruling on someone who is suddenly attacked by a person firing a weapon, and he himself is armed: should he retaliate or surrender? And if he kills the one who attacked him, is it considered intentional killing or self-defense?

The Hadith you mentioned is authentic, found in the Two Sahihs and other collections, and it indicates that if Muslims fight each other with swords, both the killer and the killed will be in Hellfire. Scholars have interpreted this Hadith as applying to those who have no valid justification for their actions.

Therefore, if someone attacks another, and the victim finds no means of escape except by killing the assailant, then there is no sin upon him for that. This is supported by what Muslim and others narrated: A man said, "O Messenger of Allah, what if a man comes intending to take my property?" He said, "Do not give him your property." He said, "What if he fights me?" He said, "Fight him." He said, "What if he kills me?" He said, "Then you are a martyr." He said, "What if I kill him?" He said, "He is in the Fire."

Shaykh al-Dardir said, "It is permissible to repel an aggressor against oneself, property, or honor." And killing is the last resort to repel an aggressor.