Jesus agreed to die for humankind because Satan told God that no perfect human would ever stay on God's side. And Satan had just gotten two perfect humans(Adam and Eve) to turn away from God and join him(Satan). So Jesus agreed to be put to the test even to death to prove Satan a liar that he is. And Satan made it rough for him but Jesus prevailed and saved humankind. Now Satan doesn't have a leg to stand on and soon Jesus will take his rightful place as ruler of this earth and get rid of Satan and the demons. That is why Jesus had to die and why the death was so brutal. Jesus could have said no at anytime but didn't and stayed true to God.