Complex sentence: Cassius Dio states that the Empress Valeria Messalina, out of fear of Pompeius being a rival to Britannicus, ordered his execution, so that Antonia could marry Messalina's half-brother Faustus Cornelius Sulla Felix.
Split into simpler sentences: Cassius Dio states that the Empress Valeria Messalina, ordered his execution out of fear that Pompeius might become a rival to Britannicus. The death of Pompeius left Antonia free to marry Messalina's half-brother Faustus Cornelius Sulla Felix.