الْقَاتِلُ وَالْمَقْتُولُ فِي النَّارِ is a grave warning. It does not justify the sweeping conclusion that every innocent murder victim deserves punishment. Nor does this short wording authorize certainty about the fate of a specific deceased person.
People participating in unlawful mutual violence are not the same as an innocent victim who is attacked. For beginners, the central lesson is the seriousness of taking life and the duty to avoid participation in violent disorder.
Example: Joining a confrontation because of an unverified rumor can put lives at risk. Verify information, avoid escalation, and seek appropriate assistance.