Seriously though, considering the evil acts of the inquisition and the inquisitors, they were a force to be reckoned with back in the day. Also, think about modern day terrorists and radicalists, they act the way they do over religious reasons.
Think of Hitler. He killed thousands of people with the great help of his words and propaganda.
And gas chambers. The most deadliest weapon would have to be the flamethrower. So deadly that they made it against the rules in war. Which is sort of ironic, rules in war.
Oh, I'd also like to add something, before people say that 'religion wasn't created as a weapon'. That may be true, indeed it probably is true, but that doesn't mean it can't be used as a weapon. Take a brick, for example, it's 'true' use is for building things but put in the right hands it becomes a weapon for bludgeoning people with (I'm not sure how putting a half-brick in ones sock and hitting someone around the head fits into this metaphor so I'll just leave that part out).
Oh, I'd also like to add something, before people say that 'religion wasn't created as a weapon'. That may be true, indeed it probably is true, but that doesn't mean it can't be used as a weapon.
Poison gas, mustard gas, Phosgene, hydrogen Cyanide that kind that was used in WW1 nasty stuff since some was engineered to smell good, so someone will take a big whiff and BAM hydrochloric acid in your lungs
My history teacher was teaching us about WWI weapons the other week, which is mainly why I brought up the flamethrower. Mustard gas had some terrible, terrible effects. X_X