Sgt Roberts, 33, from Shipley, West Yorkshire, with the 2nd Royal Tank Regiment, had been ordered to give his enhanced body armour to another soldier because of a shortage of equipment, when he was hit by "friendly fire" during disturbances at Al Zubayr near Basra.
The Iraqi, Zahir Zabti Zaher, was repeatedly shot in the same incident while allegedly throwing stones at the troops. Iraqi human rights activists claimed that he had been "executed" by soldiers enraged at the death of Sgt Roberts
...
He said Sgt Roberts was on a patrol with three Challenger II tanks when Mr Zaher began throwing stones.
After signalling him to stop, Sgt Roberts, who was standing alone outside his tank, is said to have fired one shot from his pistol before it jammed. Soldiers in the tanks opened fire and the sergeant was hit by two bullets
http://news.independent.co.uk/uk/legal/article 360666.ece
Doesn't sound much like a snap decision to me - sounds like somebody panicked and lost the plot!