There is a clear and noticeable difference between the ways in which an M16 fires these two types of ammunition. When firing live rounds, the spent cartridge is immediately and automatically ejected from the chamber at the side of the gun. When firing rubber coated bullets, the forces involved are weaker, and the spent cartridge stays in the barrel after firing. The spent cartridge can only be released by cocking the gun.
When the border policeman fired his weapon, the cartridge was ejected immediately and automatically. This demonstrates he fired live ammunition.
Firing live ammunition while there is a rubber bullet in the extension would cause a major malfunction of the weapon, a possible blast that would endanger the shooter.
Thus, for the death of Nadeem Nawara to not be premeditated, a nearly impossible series of coincidences would have needed to occur. Instead, what happened was that 1) live ammunition was fired; 2) a rubber bullet was not inserted into the extension; and 3) despite regulations to aim at the legs of protesters, Nawara was shot in the chest.
With these circumstances taken together, it is most likely that Nadeem Nawara was killed intentionally.