Meet St. Olga.
If the murderous and vengeful Cersei Lannister, from Game of Thrones, had an identical sister that was actually a real person, she would be St. Olga. There has likely never been a Saint who was single handedly responsible for more cold blooded killings than St. Olga.
Olga's story takes place in 10th century Kievan Rus, modern day Russia and Ukraine.
After her husband Igor, the ruler of Kiev, was assasinated by a rival clan the Drevlians, Olga secretly vowed revenge. Not just any revenge though...
The total obliteration of the Drevelians.
After assisinating Igor, the Drevlians pai Olga a visit and emanded she marry their Prince Mal, making him the ruler of Kievan Rus'.
Olga didn't take kindly to this.
Olga ordered her men to dig a trench. The Drevlian men were then ordered into it and buried alive.
Olga then sent word to the Drevlian Prince that she accepted his offer of marriage but she needed his twenty most distinguished men to escort her to him.
Prince Mal obliged and sent the escort.
When they arrived Olga invited them into a bathhouse to clean up and rest from their journey. Once inside, the doors were locked shut and the house set on fire.
Olga wasn't satisfied, she then set out to destroy the rest of the Drevlians. She invited them all to a funeral feast for Igor. Somehow oblivious as to the whereabouts of their missing men they attended the feast. After getting all of the Drevlians in attendance good and drunk she then had them all killed.
She then seized the Drevlian city and had their remaining people either killed or sent into slavery. The number of Drevlians too have been killed by Olga is said to be in the thousands.
Wait? This lady is a Saint?
Yeah. She turned it around later in life.
While visiting Constantinople she converted to Christianity. The specifics around it are unclear, but she did return home determined to convert her people.
She had very little success before she died in 969. She viewed her evangelization as a massive failure. Her son, Svyatoslav, the next ruler, remained pagan and refused to convert to Christianity viewing it's teachings as weak. Though the country remained pagan for the remainder of Svyatoslav’s reign, his son Vladimir did make Christianity the official religion of the nation in the 980s.
Though she would never see the fruits of her labor in her lifetime, Olga's attempts to convert her people paid off!
St. Olga, ora pro nobis!