What motivated a Jordanian soldier to murder 3 US instructors?

A Jordanian soldier, Sgt. Marik al-Tuwayha, has been sentenced to life in prison with hard labor for killing three US military trainers last November as their vehicles approached the al-Jafr airbase where they were training Jordanian army troops in counter-terror tactics. The US embassy said the trial had confirmed Staff Sgt Matthew Lewellen, from Missouri, Staff Sgt Kevin McEnroe from Arizona and Staff Sgt James Moriarty, from Texas, "had been following procedure" when they were killed. Their families suspect a cover-up.

Jordanian authorities initially claimed they were killed when their vehicle did not stop at the gate to the base.
The case raised a storm in the US and Jordan. The victims’ families charged the trial was rigged after the judge refused to allow video tapes from the base entrance showing the Jordanian soldier firing for six minutes, even as the instructors shouted, “We are Americans, we are friends.”

The Jordanian judge also refused to allow the testimony of an American solider who witnessed the incident. Support groups had meanwhile sprung up for the Jordanian soldier and his claim that he opened fire when he heard a pistol shot and was sure the base was under attack. He also said the Americans instructors refused to present identification.
A separate US military panel set up to investigate the affair established that there was no gunfire preparatory to the attack on the American soldiers and they had behaved according to procedure.

debkafile adds: This case has touched on the most sensitive royal nerves at King Abdullah’s court in Amman. Tuwayha tribe members are held in high regard as the most outstanding fighters of the royal army. Their motto is Allegiance to God, the King and the Tribe.
Sgt. Marik Al-Tuwyayha’s conviction and sentencing to a long prison sentence may weaken the tribe’s loyalty to the royal army and the king. His sentence will therefore most likely be cut short on the quiet.
The judge who sentenced him did his best to dispose of the case without exposing the motives of the accused in shooting the Americans. The soldier's affinity to the most extreme Salafi groups which have strong leanings toward the Islamic State, to the point that he was ready to murder three American military instructors, needed hushing up, lest it laid bare the extent of ISIS penetration of the Jordanian armed forces.