Final answer:
Husayn, the son of Ali and grandson of Muhammad, became a martyr for insurgents, specifically those who would form the Shia branch of Islam, after his death at the Battle of Karbala in the year 680.
Step-by-step explanation:
The death that gave the insurgents a martyr was that of Husayn, Ali's son and Muhammad's grandson. Husayn's tragic end occurred in the year 680 when he and his family were ambushed and killed, an event which is a pivotal moment in Islamic history that led to the split between Sunni and Shia Islam. Shia Muslims regard Husayn as the rightful heir to the caliphate, taken from them by the Umayyad family's ascendance to power. The martyrdom of Husayn is mourned by Shia Muslims especially on the day of his death, Ashura, marking it with processions and festivals of repentance in places like Karbala, Iraq, where the massacre took place.
There are other examples historically where the death of an individual has provided insurgents or groups with a martyr figure, such as the decapitation of British aid worker Alan Henning by ISIS, or by implication, US troops killed by insurgents during the occupation of Iraq. However, the reference to insurgents gaining a martyr particularly refers to the historical Islamic context of Husayn's death.