Of course my country's history comes to my mind first.
Maybe the scope of the murders committed by Germany was not the highest; 6 million Jews and a few million Gypsies, handicapped, homosexuals and so on.
Stalin's USSR murdered 20 million and Mao's "cultural revolution" cost the lives of 80 million.
But Germany's example still appears more cruel and extreme to me. That's because of the contrast to the high degree of culture and civilization Germany was capable of before. Don't get me wrong, I'm not saying Russia or China were not civilized. But a life had never been worth much in these countries, their general level of wealth and education had been much lower, famines and tyranny had cost many lives all through their histories, so it was not such a sudden break with a civilized and wealthy past as in Germany.
Just nobody expected a country like Germany would be be capable of the most evil and barbaric acts it committed. But educated and saturated citizen still blindly obeyed orders, became the worst kind of sadists, murdered innocent women and children and would then go back home to read their kids bedtime stories.
"Evil" history? I don't think that's even possible.
If you want to ask which country has the most morally questionable, yeah, but evil is such a broad, encompassing term and it's subjective. Because technically, I bet there's many people out there who think the US has some seriously "evil" history.