Note 13 at Acts 18:17: Sosthenes (meaning of sound strength) became the chief ruler of the synagogue after Crispus converted to Christianity. It is also supposed that this Sosthenes became a Christian because he was included in Paul's greeting to the Corinthians in 1 Cor. 1:1. This is possibly why the Jews beat Sosthenes.

If they couldn't take their wrath out on Paul, then they turned on Sosthenes who was sympathetic to Paul's teachings or else had already embraced the Christian faith. If Sosthenes was not yet a believer when the Jews beat him, maybe that was the "last straw" that turned him from the hypocrisy of the Jews to the message of God's love that Paul taught.