The opinion that Christianity is responsible for more deaths than any other religions is right. If you consider everything from the Inquisition, pogroms, the Crusades, the Holocaust, and other historical events, the numbers are certainly high. I can't think of any other group that has caused that much death over its (relatively short) history.

As to which has the least blood on its metaphorical hands, I don't know, but I would presume it's a group that has pacifism as part of its core philosophy, so Buddhism probably ranks 'way down there.

Seems to me like I read once that the First Crusade entered Jerusalem and slew every single person that didn't flee, except themselves. Allegedly, the blood ran in the streets up to the ankles.

Atheism, as endorsed by Joseph Stalin, Mao, Pol Pot, 110 million,

This dwarfs Hitler, also an atheist Darwinian architect of his Master Race.