As far as I am aware, according to Christian beliefs after Adam's fall all humanity became sinful and doomed to perdition. Then Son of God was sent by Father to save mankind and give eternal live for those believing in Him.
Why was all this needed? Why didnt God simply send a prophet with instructions how people can save themselves? Why did God have a need to make sacrifice for himself?
The people tend to turn a deaf ear to a prophet, most especially if he tells them what they don't want to hear. That is pretty clear in the Bible.
The concept of blood sacrifice was thousands of years old and deeply ingrained in the religion and culture of the people of Jesus' time. So God became incarnate in the person of Jesus and willingly suffer and died on the cross to pay in blood everybody's sins. And on the third day he rose as the beacon and assurance that death was not final, and his promise of eternal life was real.
That's pretty much it in a nutshell.