First, assuming that he actually existed, the Romans killed him.
This would have gone down well in Judea at the time where feelings about the Roman occupation were running high enough to cause revolt. Even more so after the heavy handed suppression of that revolt.
However, Paul was trying to sell his religion to the Romans. So all those dietary restrictions, clothing restrictions, working restrictions get thrown out, and the Jews get stitched up for the cause of Jesus' death, while the Romans were trying to do the best they could.
Meanwhile for the mythology to work out, then yes, Jesus has to die a brutal death. The symbology of dying in his bed, surrounded by his wife, kids and grand kids after a long life --- for our sins - does not carry the same weight. "Oh, Lord, forgive them for giving me a nice life, a wife and family and dying of old age" does not have the same ring to it.
This was the key argument in the gospel of Judas that got rediscovered a few years back. Judas was acting as Jesus directed. He had to be betrayed, and had to be crucified.
This, while being fairly obvious ( I worked this one out in my teens), went against religious doctrine, so most of the copies of The Gospel of Judas were destroyed, and the character that the early church leaders wanted was put into his place, and Jesus was turned into a victim rather than a suicide.