Who killed more people the Christians or Communists?

This really is something of an asinine question. Entire millenia have passed between the advent of Christianity and the advent of Communism. So, to even make this remotely comparable, you would probably want to compare them in the same time frame, i.e., Christians since the advent of Communism. Then, you have to consider what your basis is going to be... the Russians were traditionally Orthodox Christians, even as they committed their acts in the name of their political ideals, so you could say they were both Christians and Communists. The Second World War was fought by large numbers of people who subscribed to some denomination of Christianity, but were not fighting for a religious cause. Even many conflicts between people of various faiths aren't so much about religious principle than they are about political ideals. Take, for example, sectarian violence in Northern Ireland, where the divide has traditionally been between Protestants and Christians, but the situation itself is political in nature.

So, yes, one could say that Christians have killed more people, but, if you compare within the same time frame the number of people killed by Christians for the sake of religious zeal vs. the numbers killed by the Communists as part of various political pogroms, the Communists would have a substantially higher number to their claim. But then we would need to assess the wiping out of early Christian sects during the early years of the Roman Catholic Church, the crusades, and of course, the many Inquisitions that wiped out whole populations of innocents.