The purpose of the law was :
1) To reveal our sinful state. Rom 3:20
2) To inflame sin Rom 7:8
3) To minister death in us Rom 7:10-11
4) To lead us to Christ Gal 3:24 ( He has fulfilled the law for us..now we walk in His love which is in us and thus fulfill the law )
The law is good! But the purpose of the law was not for righteousness or salvation at all, it was to manifest sin in our lives, so that the purpose of grace which is in Jesus is to manifest salvation. Grace does not set aside the law, but completely satisfied it.
Col 2:13-15 is where in says that Christ "disarmed" satan having cancelled out the certificate of debt consisting of decrees ( the Law ) against us and He nailed it to the cross. Satan uses the law against us. Satan comes and says to you - you broke the law - you are condemned now according to the law.
BUT our Lord took that punishment for breaking that law for us. When Christ takes away the condemnation of the law which satan uses against when he accuses us - he is being "disarmed". If disarmed means anything - it means he did have a weapon but now he doesn't because of our Lord. How great is our salvation in Him!