Mr Chitnucha, was sitting on the edge of the pickup. Witnesses told police that the truck slowed down at the entrance to a soi to allow people in the back to throw water at another group of young people.
Two unidentified men on a motorbike wearing full-face helmets then reportedly stopped behind the pickup. The rider on the back of the bike pulled out a homemade gun and shot Mr Chitnucha in the back.
The bullet burrowed through into his heart and he died on the way to Vachira Hospital.
Police said they believe “personal conflict” motivated the murder. The shooter and his driver have yet to be apprehended.