Complex sentence: Some collaborated with the Pakistan Army in atrocities against Bengalis during the war; as a result, Biharis faced reprisals from Bengali mobs and militias and from 1,000 to a more- "likely" figure of 150,000 were killed during the war.
Split into simpler sentences: Some collaborated with the Pakistan Army in committing atrocities against the Bengalis during the war, and as a result, Biharis faced numerous reprisal attacks from Bengali mobs and militias. With between 1,000 to a more "likely" figure of 150,000 were killed during the war.