Complex sentence: It is likely, therefore, that Collingwood had already witnessed the mass-murder of slaves and, as the historian Jeremy Krikler has commented, this may have prepared him psychologically to participate in the massacre which occurred on the "Zong".
Split into simpler sentences: It is likely, therefore, that Collingwood had already witnessed the mass-murder of slaves. As the historian Jeremy Krikler has commented, this may have prepared him psychologically to participate in the massacre which took place on the "Zong".