Complex sentence: Decio, Cesare and Fabio murdered her tutor, who carried the letters acting as an intermediary, and Isabella was the second victim, as she was found with the letters in her hands according to the reports of the time.
Split into simpler sentences: Decio, Cesare and Fabio murdered her tutor, who carried the letters acting as an intermediary. Next they hunted down Isabella, as she was found with the letters in her hands according to the reports of the time.