These sorts of debates tend to fall into the same talking points over and over again. It's kind of like this:
Nonbeliever: Religion brought us the Crusades! And witch burnings! And flying planes into buildings!
Believer: Oh yeah, what about Stalin, Mao, and Pol Pot?
Blah blah. And of course, sometimes you get the spectacularly misinformed nonbeliever (looking at you, Bill O'Reilly) who insists that Hitler was an atheist. Pretty strange considering how much he spoke of Divine Providence and how his regime banned atheist organizations. If you want to try and tell me Hitler wasn't a Christian, that's a conversation worth having. But calling him an atheist? That's demonstrably wrong.
I already reviewed Justin McRoberts's CMYK, but in it, he brings up that humanity doesn't need religion to do awful things. It's a good point. He even asserts that things like slavery did not come about because of religion, and I totally agree with that as well. Nobody needed a holy book to exploit people.
Of course, the flip side of this is that if people don't need religion to do bad things, they don't need it to do good things as well. Some of the world's prominent philanthropists are atheists, so no religion required for that. Also, if you're religious, I'm sure that your atheist neighbor will be just as likely to do nice things for you like give your car a jump start as your Sikh neighbor would.
So, what am I saying? Is religion neither here nor there on the good/bad scale?
No, but religion emboldens ideas, whether they're good or bad. If you think that Jesus wants you to do good works, that's not necessarily going to be the reason why you do good things, but it helps you to feel justified in what you're doing. However, you can turn this around, and it can also lend an unearned credence to horrible ideas.
It's true that religion didn't create slavery. However, it sure as heck helped give some motivation to those who perpetuated it. In the Christian world, the fact that The Bible condones slavery sure helped those who wanted to keep it going. (And please, don't write to me saying that it doesn't and give me crap about "context". This is a stupid argument. It specifically says that you can beat your slave as much as you want just so long as you don't kill him. There is NO CONTEXT in the world that can make this anything other than an endorsement for slavery unless the very next line is "Scratch the previous sentence, 'cause that's plain evil. Just checking to see if you're paying attention.") Yes, I am aware that many prominent abolitionists were Christian and claimed to oppose slavery out of their religious convictions, but again, the flip side of this is true, unless you want to try and tell me that there aren't a lot of Christians in the South.
The thing is, you can have bad reasons for doing things that are good. If I donated money to help fund prostate cancer research because I thought that if I didn't, gremlins would eat my earlobes, the fact that I'm doing something good wouldn't somehow justify my belief in gremlins, would it?
The problem then is not religion, but a lack of critical thinking and proper skepticism. This addresses pretty much every religious atrocity, from large-scale horrors like The Holocaust (don't tell me that wasn't emboldened by religious beliefs when you have over a 1000 years of both Catholicism and Protestantism scapegoating the Jews as "Christ Killers") to smaller-scale atrocities like parents who don't take their kids to the doctor because they think that Jesus doesn't want them to do that.
This also confronts your secular abominations like what happened in The Soviet Union under Joseph Stalin. Believers love to throw that one around. "It was all done in the name of atheism, and they killed more than anybody else!" While it's true that they were atheists, or at least, they preached an atheistic doctrine, I should also point out that The Crusades would have probably claimed a lot more lives if they had the same technology as the Soviets did. But back to my point, while it may have been an atheistic regime, guys like Stalin weren't trying to enforce skepticism and critical thinking. Under that program, Stalin's word was law, and he was not to be questioned. Atheism? Sure. Skepticism? Quite the opposite.
If people were encouraged to think critically and skeptically, we would still have bad ideas, and we'd still have people doing evil things. However, those bad ideas would be subject to question, because with skepticism, there are no sacred cows, and EVERYTHING is open to debate. People don't get to point to their Bible to justify everything from opposing same-sex marriage to handling poisonous snakes. And nobody gets to come forward and say that his or her ideas are automatically better and not to be questioned, whether he claims to be speaking for Jesus, Shiva, or The Fonz. Religion might not necessarily be bad, but it's often a conversation stopper when one can just point to a divine authority and say: "It's the will of Zeus!"
Religion's not the problem, it's just a symptom of the real problem: the lack of critical thinking and skepticism.
And I realize that there might be some Christians (or other theists) out there who are saying, "Hey! I'm a Christian, and I'm a skeptic, too!" While probably worthy of another blog post, my quick response is:
No. You're not. If you believe that a guy died on a cross and came back three days later, and your basis for this is either your subjective, personal relationship and/or a series of writings that were written at least a couple of generations after the supposed event took place, then you are NOT a skeptic. I'm sure that you're a wonderful person in every other regard, but you can't have your Jesus and critical thinking too. (Sorry to pick on Christians. Substitute any other supernatural event and/or belief system that's been debunked and it works just as well...except for Shinto, that stuff's legit.)