a Herodian named Saulus accompanied the Romans, cross-examining prisoners and negotiating on behalf of the Romans. Robert Eisenmann believes that this was Paul of Tarsus.
Relatives of Herod were a small group, much less those named Saulus who were cozy with the Romans. Paul's letters also appear to show increasing anger and alienation regarding the Jews, even if they contain some later emendations.
If Paul had been martyred, his martyrdom would have been memorialized like Peter, James, and many others. If Paul had been set free, that also would have been undoubtedly taken as a sign of God's providence -- it is also extremely likely that subsequent stories, letters, or sayings of Paul would exist after his release.
Paul becoming a Roman agent, however, would have been an embarrassment, and there tends to be a silence around embarrassing facts. For example, when Shabbatai Tzvi claimed to be the Jewish messiah in the 17th century and subsequently converted to Islam, his memory was all but buried.
It's unlikely that Paul's fate will ever be proven. This is the only version of events, however, that explains the silence.