Those sins were committed by men as you say, not God or gods so why a requirement Christ be both man AND God to be a sacrifice?
Indeed as Paul says, ‘If death is through a man, resurrection of the dead is also through a man’.
That’s a good question.
Sin is not just between men and men. When I sin the sin doesn’t just affect me and the person that I hurt. Sin also affects God. God despises sin.
If God is just then there must be the appropriate penalty for sin. Since the Fall, we cannot save ourselves, we have become corrupted by sin, and we need God in order that we might be saved from Sin.
This is why God, as Christ the Son, offered himself as a sacrifice so that he would be the passover lamb without blemish, in fulfillment of the prophecies from long ago. Fully God and fully Man, he is the bridge between man and God such that our sins have been atoned.