The legalist wanted the Galatians to add works to their salvation, but salvation plus anything else leaves out justification by faith in Jesus. Just as a master of a slave wants to keep a free slave bound, the legalist want to keep you in bondage. We are free in Christ and the law is fulfilled by loving God and loving our neighbors through Christ.
We should not use this liberty as a license for sin. Galatians state that those who practice the lusts of the flesh will not inherit the kingdom of God. This practice is a habitual practice. Romans 6:6 says that we are no longer slaves to sin. If the Holy Spirit lives within us, we will not fulfil the lusts of the flesh. Instead we will have the fruit of the Holy Spirit. The Holy Spirit will never lead us into sin.