Complex sentence: Macreedy warns Smith that he knows Smith killed Komoko, but was too cowardly to do it alone, so he had to bring Hector, Pete and Coley Trimble into it.
Split into simpler sentences: Macreedy warns Smith that he knows Smith killed Komoko, but, he continues, Smith's mistake was being too cowardly to do it alone. He had to bring Hector, Pete and Coley Trimble into it.