Cain killed his brother who was obeying God.  God said to Cain, “What have you done? The voice of your brother’s blood is crying to Me from the ground” Gen 4:10.  Jesus was killed for obeying God.  His blood also cries out.  “And to Jesus, the mediator of a new covenant, and to the sprinkled blood that speaks a better word than the blood of Abel” Hebrews 12:24 (NRSV).  The speaking blood of Abel cried out for justice for the murderer.  The speaking blood of Jesus cried out for mercy for the very murderers who shed it.  For those who have fled to Christ, the blood speaks remission of their sins, declares sinners to be children of God and speaks full reconciliation to the Father.