Hi ChosenYour Galatians 3 is the best way of viewing when exactly Christ became sin for us; v13 says 'Cursed is every one that hangeth on a tree'. In Gethsemene the shadow of the cross fell upon Him and His distress was unbearable, but the sinbearing took place as he 'hung on the tree'.
What killed Jesus, heres my answer.Understanding that Christ was a sacrifice for humanity. In Hebrews it explains this, Christ was the final and greatest high priests. He offered up himself. He did this to appease a God who demanded holiness.The answer to what killed Jesus is him offering up himself as a sacrifice to God and God pouring out his wrath on Christ who at that moment took on the sins of the world. God's wrath, justice, and holiness demanded that sacrifice and Christ appeased that wrath (for those who believe).Its amazing how God can be both loving and wrathful, what killed Jesus was Christs love and God's wrath.