Treason    verb = to commit treason
Committing treason is a serious crime in every country in the world, just as it was at the time of Jesus. The crime includes any speech or action which helps in the unlawful removal of the government or ruler.
    
Tiberius, Roman Emperor from 14 - 37 CE
As far as the Romans were concerned, they were the lawful rulers of Palestine at the time of Jesus, because they had invaded and conquered it in 63 BCE. Naturally the Jewish inhabitants disagreed and there were many who rebelled against the occupation of their land. One particular group was called the Zealots who tried to cause as much disruption as possible by attacking Roman soldiers at every opportunity.
The Romans had eventually removed the Jewish King (Archelaus) and replaced him with a Roman Governor or Procurator, who ruled the country on behalf of the Emperor. The Governor's main job was to keep order among the people and make sure that taxes were paid regularly to Rome. He used the Sanhedrin to help him keep order, and appointed its High Priest.

Palestine was a difficult country to occupy, so fierce was the opposition, and by the time of Jesus' death they were onto their fifth Governor, Pontius Pilatus (Pilate). He would be interested in executing anyone who might encourage rebellion or who was a threat to Roman rule.

Pictured right is a Palestinian stone block bearing the name of both the Emperor Tiberius and the Governor Pontius Pilatus


When the Sanhedrin wanted Pilate to execute Jesus for his blasphemy, they had to change the accusation so that Pilate would see Jesus as a danger to his rule. Many Jews were looking for a leader to drive out the Roman occupying forces. Jesus' entry into Jerusalem on a donkey on the Sunday before his death, and his actions against the Temple traders led many to see him as a Messiah who might lead a rebellion against the Romans. One of Jesus' twelve disciples was, apparently, also a Zealot (Simon the Zealot).
The possibility that Jesus was a Messiah or King and had caused a disturbance in the Temple was certainly something which would worry Pilate. Pilate eventually ordered Jesus' execution for treason. The crime of which he had been convicted was written on a sign above the cross in Hebrew, Greek, and Latin, so that no one would be in any doubt as to why he was being executed. The sign stated the criminal's name and his crime:
Jesus of Nazareth King of the Jews

© Mr.B at Farcaster Communications
The conviction notice is often shown on crucifixes today as INRI which stands for the Latin:

Iesus
Nazarenus
Rex
Iudaeorum

(N.B. There is no J in Latin)

In the Hebrew on the sign the
word Rex (King) would have read
Moshiach = Messiah.