The Old Testament Law was the old covenant for the Jews. Since Jesus rose from the dead we're all subject to the new covenant of grace. That's why someone could be stoned to death for breaking the Sabbath yet Paul says we don't have to keep the Sabbath unless we want to.
RE: I am very confused by Galatians 2:21 which seems to directly contradict Deuteronomy 6:25: "And it will be righteousness for us, if we are careful to do all this commandment before the Lord our God, as he has commanded us."