Does the fifth greater sin, Murder, exclusively pertain to the killing of a Muslim due to their beliefs, or does it encompass other motives as well? What defines 'mischief-making' in the Quran that could justify a Muslim killing another Muslim?
Killing an innocent is a major sin if it was deliberate not unintentional. This major sin has many types and degrees but all deliberate killing of an innocent is a major sin. Killing a believer just because of his belief is the most dangerous.
Wassalam.