Complex sentence: Rossa had observed the distribution of BR propaganda and had reported those involved to the police; he was shot and killed by the Brigades, but this attack against a popular trade union organizer totally alienated the factory worker base to which the BR propaganda was primarily directed.
Split into simpler sentences: Rossa had observed the distribution of BR propaganda and had reported those involved to the police. He was shot and killed by the Brigades, this attack against a popular trade union organiser proved disastrous, totally alienating the factory worker base to which BR propaganda was primarily directed.