Complex sentence: Unfortunately when they came out the situation devolved into chaos, owing to the rash behavior of the head of the BRB, Raymond Mertz, who started a shootout, in which Bruno Berliner and a police officer, Jean Vrindts, were killed.
Split into simpler sentences: Unfortunately when they came out the situation devolved into chaos, owing to the reckless action of the head of the BRB, Raymond Mertz. In the ensuing shootout, one of the gangsters, Bruno Berliner, and a police officer, Jean Vrindts, were killed.