Or to ask it simply - which Rome killed more people for their beliefs, polytheistic or monotheistic Rome?
An odd question as it's worded. How are you defining monotheistic Rome? Constantine through the fall of Constantinople? Constantine through 476? And killed purely on account of religious beliefs? If you could PM me the original link I may be able to more helpfully answer the question.
A side question.
Does anyone have any good reads on what religion played in warfare during the conquest of the Gauls?