While I am not a defender of religion and its supposed benefits, I also would not say it's "the worst thing that ever happened to us." At some point in the early intellectual development of humanity, when we first began to try to understand the world, it was almost inevitable that one form this would take would be religion. And it may have been a useful thing back then, who knows. Of course, we are far past that point now, and we have much better ways to understand the world. Hanging on to religion now is like a teenager who still sucks on a pacifier and won't give up his baby blanket.
One of the worst things that ever happened to us was that, as part of our social nature, we developed this idea of conformity, that it's a good thing to convince or force others to act or think as we do. This has become embedded in some (not all) religions -- those religions that believe it is their "duty" to proselytize and gain converts. And while that can be done in a peaceful manner, it can also take the form of violence all too easily, and this has been a huge source of strife throughout history. Actually it still is. In many islamic countries, for a native of the country to renounce islam means a death sentence.
Again, I'm not saying this is an inherent quality of religion. I'm saying it's an inherent quality of our deep urge to conform, and to make others conform. This also applies to other areas like politics.