Who died first, Jesus or the thief?
"Therefore, because it was the Preparation Day, that the bodies should not remain on the cross on the Sabbath (for that Sabbath was a high day), the Jews asked Pilate that their legs might be broken, and that they might be taken away. Then the soldiers came and broke the legs of the first and of the other who was crucified with Him. But when they came to Jesus and saw that He was already dead, they did not break His legs" (John 19:31-33).
Jesus was already dead, so his legs were not broken. The thieves were still alive, so their legs were broken to hasten their deaths.
So the new covenant had been established before the thief died? So the thief was not under the law of Moses?
At the time the Son of God told him he would be in paradise that day, the thief was under the Old Covenant. His forgiveness came prior to the establishment of the New Law. Since he would not have had time to commit additional sins, the thief would remain saved even though the laws changed just before his death.