What is the detailed explanation of the Hadith "If two Muslims meet each other with their swords, then both the killer and the killed are in the Hellfire," and the interpretation of why the killed one is in the Hellfire, along with mentioning the different situations in which this Hadith applies, such as war, civil strife (fitna), and assault?

The agreed-upon Hadith (definition) that "the killer and the killed are in the Hellfire" is understood to apply to fighting between Muslims without a permissible justification, such as tribalism or the like, and not to every form of fighting. Fighting with a permissible justification is not included in the Hadith, nor is fighting in self-defense, or in defense of one's property, or religion, or family. Whoever is killed in defense of these is a martyr, and whoever transgresses, their killer is in the Hellfire.