Spanish Inquisition
If you want a modern day equivalent...
Who is the person whose single handed action has lead to the death of the most people?
Someone who banned the use of Condoms as a lowly Vatican official, continued the ban whilst in office and ensured that all Priests in Africa told their flock that using a Condom would lead quickly to hell. And the Catholic Church went and fast tracked him to Saint, when at least 30M people have died of Aids due to unprotected intercourse.