Now that Paul has given the business to the Gentiles in Chapter 1, now he takes out his "butt kickin'" stick for his Jewish brothers and sisters. The whole point of chapters 1:18-3:21 is that ALL people are sinners and, therefore, all people are on a level playing field.
Remember that the Jewish-Christians had been banished from Rome for roughly 5 years and returned to find the non-Jewish Christians running the churches that they had founded. Add to this fact that many early Jews, as attested to by the Book of Acts, felt that the Gentiles were still second-class members of the People of God.
Paul argues that at this time in Salvation History (and until the return of the King), that all people have sinned and deserve an eternal death penalty but the great Apostle to the Gentiles will not leave all people roasting in the pit...he's about to set forth the solution...the only solution.