Note before Paul spoke of Galatians 2:17, he rebuked Peter separated himself from the Gentiles when Jews fellows arrived, for Peter was afraid of those who belonged to the circumcision group (Gal 2:12).
Paul's belief is, as he wrote in Ephesians 3:6
6 This mystery is that through the gospel the Gentiles are heirs together with Israel, members together of one body, and sharers together in the promise in Christ Jesus. (Ephesians 3:6 NIV)
Paul said: "a person is not justified by the works of the law, but by faith in Jesus Christ" (Galatians 2:16 NIV), so it is not the Gentiles follow the practice of the Jews to become righteous, it is the Jews should quit their practice and follow the Gentiles putting their faith in Christ.
Return to Galatians 2:17
17 “But if, in seeking to be justified in Christ, we Jews find ourselves also among the sinners, doesn’t that mean that Christ promotes sin? Absolutely not! (Galatians 2:17 NIV)
Paul said: "no one will be declared righteous in God’s sight by the works of the law; rather, through the law we become conscious of our sin" (Romans 3:20 NIV). It means those who practising the law still see themselves a sinner. Does it mean Christ promotes sin such that His coming had a meaningful purpose? Absolutely not! For sin always exist.
As it is written: “There is no one righteous, not even one; (Romans 3:10 NIV)
Christ is not coming to promote sin, the Jews should know that "a person is not justified by the works of the law, but by faith in Jesus Christ. (Galatians 2:16 NIV)
Bearing the above in mind, let me answer the two questions in OP's last two paragraphs.
We have to understand that we are no longer a sinner once we put our faith on Christ. For we believe His crucifixion, buried and resurrection already redeem our sin. Therefore Jesus is not coming to serve sin, but give life. However, we must hold on to a new life by keep the faith, for it is possible of losing the grace.
To your last question, if someone disclosed himself once been a thief in the evaluation process, it is certainly an noble act, honesty behaviour. Surely it is just a beginning of good faith, but certainly not a perspective of condoning crime.