What's the most evil thing you've ever done in a video game?
In recent memory, I'd say the conquest of Algier in a campaign of Crusader Kings II.
I had started as the still fledging House of Savoy, and after three centuries of bribes, private wars started on dubious claims, and crusades which had little to do with religion and everything to do with ruthless expansionism, my smallish duchy had become the large Kingdom of Burgundy, a Kingdom which did not merely include most of the historical Kingdom of Arles, but also most of the Italian Peninsula, and a good chunk of the north african coast. Ruling over one third of the Holy Roman Empire subjects and nearly half its monetary wealth (my vassal the maritime republic of Pisa had a commercial empire which went from Constantinople to Ireland), my character easily forced the weak Nakonid emperor to formally aknowledge the third Kingdom of Burgundy's independence. But the Holy not-so-Roman-anymore Empire still had one province I wanted for my realm: Algier, which had belonged to a cadet branch of my House before the burgundian independence.
The war for Algier lasted a decade, cost the lives of half a million soldiers and God knows how many unharmed peasants and artisans were killed in the ceaseless plundering: see: I controlled Kabylia, so while My troops were conquering Algier, the German troops attacked and plundered Kabylia before attacking my troops in Algier. Being victorious in Algier I would then send my troops back in Kabylia to retake the province, but while they where busy there, german reinforcements came to Algier, wrestled the province back from me before attacking Kabylia. Being victorious thanks to reinforcement of my own, I would then assault Algier once again, and this back and forth game lasted one decade, turning Algier the White into Algier the Red given that, after so many army fights, plunder, retaking and replundering, I doubt a single wall of the city wasn't soaked with the blood of the locals or some invading soldier.
Oh, and of course, there was a long border between my realm and the German Empire, that ran for hundreds of kilometers along the Alps... Suffice to say, every village in southern Germany, German-speaking Switzerland and Austria was burned to the ground thrice over before the war ended.
Taking all into account, a petty war for the control of a coastal city and its surrounding lands ended up causing more death than the mongol invasions of Russia and Mesopotamia put together.
Paradox: the video game company that taught me that immortal rulers are sick bastards.