Possible meanings are 1) the Gentiles beat Sosthenes in the court in front of the judgment seat because he was the Jewish leader or 2) Sosthenes was a believer in Christ, so even though he was "the ruler of the synagogue," the Jews beat him in front of the court.
# they all seized
This may be an exaggeration to emphasize the strong feelings the people had. Alternate translation: "many people seized" or "many of them grabbed" (See: [[rc://en/ta/man/translate/figs-hyperbole]])