Jesus was killed because he was a political threat.

Rome ruled over Judea and Pontius Pilate was the Governor but Rome allowed the local municipal governments to stay in place. Rome primarily was concerned with keeping military control of the region and collecting taxes. The Sanhedrin/ High Priests were the Jewish leaders and administrators.

Jesus had primarily pissed off the High Priests in most of the New Testament by preaching in the temple, throwing out moneylenders, throwing out doctrine like working on the sabbath and so forth. The High Priests would have been eager to stop him for two reasons. 1. To prevent the Jewish people from listening to him and rebelling and removing them from power. 2. Prevent the Romans from removing them from power for not keeping control over the people and/or attacking the Jewish people in prevention of an uprising.

But here's one way Jesus would have pissed of the Romans. During Passover week, the population of Jerusalem doubled as Jews came into town to celebrate. The Governor, Pontius Pilate, lived in Caesaria, but during passover week would travel to Jerusalem with an army and make a show of force to remind the Jews that they were occupied. As you can imagine, the Roman army would have been a very impressive sight and would have been very intimidating. They entered Jerusalem from the west at the main city gate and would have marched through town to the fortress. Leading the march would have been Pontius Pilate most likely riding a big fucking horse.

On the very same day that Pontius Pilate marched into town, Jesus paraded into Jerusalem from the eastern gate, riding a donkey.

Read that again and see that image in your head. Pontius Pilate, the occupier, on his stallion with his army marching into town from one gate in order to threaten any rebels, and Jesus, a lowly carpenter who spent all of his time with the poor and diseased, parading through the opposite gate on a donkey as the King of the Jews. That's pretty fucking funny and very ballsy.

This was followed by Jesus cleansing the temple and pissing off the High Priests, and later the people turning against him for being a troublemaker. There's a whole lot I'm not writing about that happened during this week and much more needed to understand the historical context as well as why the whole parading on a donkey was so symbolic but an important point to understand is that things were very tense. And that everything that Jesus did was not only religious teaching but also political action. He was at the bottom of the totem pole but his actions were such a threat that Rome, the Jewish government, and the Jewish people all turned against him and killed him.

tl;dr: Jesus had some big ass balls