Police said Sulkif Litahae, 45, was shot dead on the Pattani-Narathiwat Road in Moo 5 village in Tambon Rusamilae in Muang district at 7.50am.
He was wearing a bulletproof vest but the bullets struck him in the head and eye.
The insurgents took away his AK-47 assault rifle and .38 revolver.
Police said Sulkif, who was riding his motorbike, was heading to the Municipality 1 School about 3 kilometres away to guard it. The defence volunteer was fully armed for his security job.
Police said two insurgents on a motorcycle raced after Sukif until they reached the scene of the incident, where two other insurgent were waiting at the roadside.
The waiting insurgents fired an M16 assault rifle and an AK-47 assault rifle at him, causing his motorcycle to fall. The insurgent riding pillion then shot him in the head at pointblank range.
Police found 10 spent shells of M16 ammunition and 6 six spent AK-47 shells at the scene.