It's based on Saint John's teaching about sin that causes death:

"If you see any brother or sister commit a sin that does not lead to death, you should pray and God will give them life. I refer to those whose sin does not lead to death. There is a sin that leads to death. I am not saying that you should pray about that." 

When before Pilate, Jesus answered: "You would have no power over me if it were not given to you from above. Therefore the one who handed me over to you is guilty of a greater sin."

Though Pilate knew that Jesus was not guilty of any crime deserving death, he nonetheless weakened and turned the Savior over to the Jewish mob. The text clearly indicates a greater degree of culpability and sin on the part of Caiaphas.