Possible motives for the killing include a desire for revenge by Qaddafi loyalists or antagonism toward the secular transitional government by Islamist extremists. Mr. Droui had joined the rebel’s transitional council when most of Surt, his hometown, remained stalwart in its support of Colonel Qaddafi, another son of the city. In the final weeks of the revolt against him, Surt was the site of some of the most horrific violence in the conflict, including at least one massacre of dozens of unarmed Qaddafi supporters, as well as the assassination and disfigurement of the colonel himself.

Since then, however, Surt has joined Benghazi and Darnah as theaters of operation for armed Islamist extremists. Some of the fighters from neighboring cities who occupied the city at the end of the rebellion reorganized themselves into the core of a militant group, Ansar al-Shariah of Surt, an independent organization that shares its name with counterparts in Benghazi, Darnah and Tunisia. (Ansar al-Shariah means supporters of Islamic law.)

Tensions are rising across the country ahead of elections for an assembly to draft a new constitution. Mr. Zeidan recently survived a no-confidence vote by the transitional Parliament, which would have required a two-thirds majority, and he promised to replace members of his cabinet. But much of the public has lost trust in Parliament, whose members were elected in 2012 but have struggled to exercise authority over the country. Parliament once planned to transfer power to a new elected body by this February, but there is little chance of meeting that deadline.

Regional groups in eastern Libya and tribal groups in the south and west have cut off or disrupted supplies of oil and gas. And over the last two days, fighting has erupted between rival tribes competing for control of smuggling routes around the southern city of Sabha; dozens have been killed so far.