Jesus was executed because he was becoming troublsome to the ruling elite of his day.
Then, as now,every action by man has to do with power.
In Christs day, if someone upset the authorities...Orthodox jews in his case,they convened a "kangaroo court" and executed them.
Nowadays they usually stop at character assasination on people who protest too much or rock the system.

The higher authority ,(the Romans) were quite happy to keep the religious /political status quo ,so didn't intervene in what they saw as a minor squabble.

The supporters of Christ expected some major event to take place, in which the forces of authority would be crushed and the "Messiah" triumph.

When of course this didn't happen, the Christians...just like the Democrats, tried to put the best possible spin on it.
This spin involved the promise of "everlasting life" if one put aside reason and swallowed the fable....an excellent tactic given mans egotistical desire to evade a natural death.

Its all very simple really...   Just read the bloody book!!