The reason that Paul would make himself a transgressor by returning to the law has to do with how he was set free from the law when he came to Christ. It involved him dying to the law which was a tremendous advantage to him, for it put him in a position where the law could no longer accuse him of sin, because as far as the law was concerned he had died.
It should be obvious that when Paul uses the term ‘law’ here, he is not speaking of the ceremonial law, but the moral law. The law he has in mind is a law which is theoretically capable of justifying. In practice the law justifies no one except Christ, because only he could meet its conditions – ‘The man who does those things shall live by them’ – but it sets forth the same terms of life to all. Paul views the law here as a covenant under which Israel hoped to obtain righteousness before God. Why else are law and faith contrasted with each other, and why are both viewed as means of obtaining righteousness and justification with God? Paul first uses the term justification in verse 16 of this chapter and compares justification by the law with justification by faith. This language is too far reaching to refer only to the ceremonial law, which never claimed to be able to justify anyone – when Christ wants to show a man that he lacks righteousness, he does not set before him the ceremonial law, but the moral law (Mark 10:19). Nor could Paul be said to have died to the law through the law, if he was only referring to the ceremonial law. The ceremonial law had no ability to bring about our death, but the moral law certainly brings death on all those who break its commandments.
How did Paul die to the law? Some understand this in a subjective sense. The full thrust of the law became apparent to him, and the sentence of death was passed in his heart as he saw the impossibility of successfully keeping it. Paul would then be saying that he died in the sense that he was driven by the condemnation of the law to seek refuge in another, that is, in Christ. If this is what he means then it is similar to the thought that comes later in the epistle in Galatians 3:24. The law is our schoolmaster to bring us to Christ because it offers us no alternative but drives us to Christ for refuge. This is no doubt true, but it would not have been a death that released him from the law, but one that condemned him while still under the law, and no one has the power to release themselves from the law.