Amjad Mohammed Ben Sasi had allegedly blasphemed during a dispute with a neighbour in the coastal city of Sirte.
He was consequently hauled before an ISIS Sharia court three times and on each occasion the youth stood his ground.
'Amjad was a proud and angry young man,' Salah Salem Ben Sasi, the teenager's uncle told The Times.
'He was fed up with ISIS rule in Sirte. His attitude was "Do what you like, I am not apologising".'
But he paid the price for his defiance and on the fourth day was brought into the main square where the blasphemy charge was read aloud to the public.
Amjad Ben Sasi remained resolute to the end, and when he was asked if he had any final words he turned to the man who was about to kill him and said 'My name will live longer than yours', the uncle recalled.
'The executioner replied, "We shall see", and shot him twice in the back of the head.'
The man then shot dead a second bound prisoner, a former policeman.
Amjad Ben Sasi was murdered in December but his body has yet to be returned to the family for burial.
Salah Ben Sasi recounted how ISIS responded to the family's many requests for his corpse by saying Amjad Ben Sasi had no right for burial as he was a non-believer.
ISIS took control of Sirte last Spring - the capital of the terrorist group's three self-styled Libyan provinces.
While many civilians have fled, the number of fighters in the area has escalated from 3,000 to as many as 6,500 in just over three months, according to the UN and the Pentagon.
It's said the recruits come via the desert road from Khartoum but only 30 per cent are Libyans.
The rest are Arabs as well as men Chad, Niger, France and India.
Initially when ISIS tightened their grip on the region in 2015 they left the locals to their own devices and kept their distance.
New legislation banned smoking and music, closed women's cosmetic shops, made the face-covering niqab compulsory for women and taxes were enforced across a wide range of businesses such as property rental and farming.
But while people were flogged or fined, executions were uncommon.
However in autumn last year more oppressive enforcement began following the arrival of an Isis leadership cadre from Iraq and terrorists confronted leaders from the city’s Salafi community and hostile members of the local al-Furjan tribe.
Public killings have become a regular occurrence, with the dead (either shot or beheaded) strung up, or left to hang on ropes from beneath a bridge on the south side of Sirte.
Many families suffer the same woe as Amjad Ben Sasi's when they are prevented from collecting the body of their murdered relative. Some even resort to smuggling the corpses away in the night in order to bury them.
Salah Ben Sasi for one is envisaging revenge once Sirte is recaptured, Daily Mail reported.
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