Three reasons:
1) to show that his love for us was greater than any amout of suffering to demonstrate that Christians should love all people as God (the Father) does
2) to show that he did die and resurrected himself as a demonstration that the body is an important part of the resurrection
3) to show that no human killed him (what he went through would have killed a true human)
He was killed by neither Roman or Jewish Priest (though I agree that they would have if they could have). He died of his own free will to fulfill the promise of the Father to all His children that a way back would be provided.
Very best regards,
Bob, Alice, and Eve
™ Version 4.2
