Not only was killing Jesus a sin, sin was already forgiven by faith. Jesus didn't teach "Christ and him crucified." He taught the gospel of the kingdom of heaven.
Jesus teaching about being rejected and killed:
‘They will respect my son.’ But when the vinedressers saw the son, they said among themselves, ‘This is the heir. Come, let us kill him and seize his inheritance.’ So they took him and cast him out of the vineyard and killed him. “Therefore, when the owner of the vineyard comes, what will he do to those vinedressers?” They said to Him, “He will destroy those wicked men miserably and lease his vineyard to other vinedressers who will render to him the fruits in their seasons.” Jesus said to them, “Have you never read in the Scriptures
Killing Jesus was wicked, but people of the Pagan atonement doctrine need Jesus killed.
You're just about as ignorant as they come!