Jesus Son of the Father (God) taking the Place of Barabbas son of a father (Satan) ... the righteous dying for the unrighteous mankind. 

Jesus was therefore, to BECOME THE CURSE by hanging on the tree - in place of the condemned mankind who had become one with the enemy SATAN after Adam and Eve sinned. 

Jesus who was without sin, by accepting to take upon Himself the sin of mankind meant that He would become a CURSE (SIN). 

Jesus became SIN (CURSE) so that we SINFUL MANKIND would become the RIGHTEOUSNESS of GOD. THE CURSE BECOMES A BLESSING!