Jesus died for our sins, and we are responsible for His death. We are born in sin. Our sins had to be paid for and Jesus had to concur death for us to live forever with Him. God's holiness required Jesus to die for our sins.
The person most responsible for Jesus death must be Adam. Adam is the first man where sin originated. The sin are transfered through the bloodline of Adam to us. As Jesus was born from the virgin Mary without the intervention of a man it meant that Jesus was free of sin. Only a man free of sin could die for the sin of man. That man was Jesus the Son of God.
He is asking His Father to forgive us for our sins that we do knowingly everyday. We should therefore strive to overcome sin every moment of the day if we love God. His love for us is so great that he gave his Son for us to live, without us understanding and comprehending what it really means.