Why did he need to do that?
Sacrifice implies that a lesser individual is giving something of value to a more powerful individual in exchange for something.
In most Christian traditions, Jesus is a component of God(God the Father, God the Son, God the Holy Spirit). If we take what Jesus is supposed to have done as accurate - who is he being sacrificed to? If we accept that Jesus and God are two parts of the same deity, then Jesus is "sacrificing" himself to himself so that he can forgive people (if they ask nicely).
The other implication is that there is a higher power - higher than this God fellow referenced in the Old or New Testaments - that required the sacrifice of a deity to give God the Father the power to forgive people's sins.
The sacrifice of God the Son to God the Father was necessary because without it there was no what? Forgiveness? Sacrifices were made in the Temple for the forgiveness of individual sins for individuals prior to and after this alleged incident. The idea being put forward is that Jesus' sacrifice was to be the last, after which there is no further need for sacrifices. If you are putting out the idea that prior to Jesus' sacrifice there was no forgiveness of sins, then you should start explaining why every person who lived prior to the Crucifiction was condemned to Hell - including all the honoured patriarchs like Adam, Abraham, Isaac, Joseph, Jacob, Moses, Samuel, etc.
Why is it necessary for God to sacrifice himself to God for people to be decent to one another?