Historically, it's very likely that Christianity has caused the most harm to human beings and humanity as a whole thus far.
If you think about the dark ages, witch hunts, torture during the inquisition, crusades, sanctions against unbelievers, such as expulsions and acquisitions of their assets, forced conversions, colonization, abuse and culture shock of natives...
Most of this bad Karma was accumulated in the early to late middle ages, until the age of enlightenment finally put a stop to most of it. Of course today you still have popes condemning contraceptives, abortion and other modern solutions to age old problems, which is still creating a considerable amount of hurt.
Then of course you also also have fundamentalist christians, such as the "God Hates Fags" nutjobs, dishing out a sizeable portion of distress to some people, even today.
I think Islam still has some catching up to do if it wants to reach christianity's accumulated level of harm. But I have to admit that current Islamists are trying pretty hard at the moment. If they go on like this, they might catch up in only a century or two.