One could also argue that they would not have been able to kill 100 million people if they believed in any level of human decency at all.
Let us contrast those guys with the Inquisitors, who killed plenty (and would have killed more, had they the power), even though they were as devout as can be. Or our current Muslims, who are the opposite of atheist, yet have no problems killing all the infidels they can get at. Neither of these groups believed/believes in evolution, yet that didn't/doesn't stop them. The only difference from the Communists is in scale.
No, I'm afraid that trying to pin human evil action on evolution won't fly.