I think Daishian's response is pretty accurate. Religion is neither good nor bad. Many people however are bad and those with a strong personality tend to rise to the top. Those people subvert religion, and most anything else by the way, to gain power and get their way. So it's humans that can make religion bad. On the other hand, Western religion is what has given us the morals that have managed to keep us from destroying humanity… although just barely.
This take on it reminds me of the gun debate and don't worry this isn't off topic its just to illustrate my point. The main question is if religion is bad for society. If religion isn't neither good or bad then how do you answer? See, its like the gun debate. Guns aren't good or bad its the person behind the trigger BUT if you give guns to a society will the outcome be good or bad? So what I'm getting at in this long ramble of words is if you put religion on society is the outcome going to be a good or bad one? Here the question already says its bad we just need to know why. Lots of posters already said many things that are true.
Religion is often bad when it takes human life as less important than the religion itself. For me religion is supposed to create human bods and help us see our potential to be civil and orderly. If a religion tells you to kill, plunder and that you are far better than another human being then its very bad for any society.
Religion can be bad for society when people use it to blindly accept what they are told instead of gathering facts and making opinions on their own. Individuals will just believe what they have been told their whole lives from their family and church instead of taking the time to question some of those beliefs and deciding if they really believe that or not.
Religion is always going to be part of human society. We are storytellers and religion is a set of stories that helps people understand the world (Not often accurately but that doesn't always matter) and their place in society (Accuracy is relative in that case). If the stories are so at odds with physical reality as to be destructive, then religion causes problems. Several very popular world religions fall in that category because they inaccurately state that the world was made for humans and that humans were made to rule it. As we act out that inaccurate story, we create misery for ourselves and others. But the origins of the inaccurate story make sense. A few cultures became very dominant because of the circumstances under which they arose (See Jared Diamond's "Guns, Germs, and Steel." As they conquered, they made up or molded inaccurate stories to explain and justify their conquest and to make it seem like it was divinely ordained. These stories form the basis for major world religions at the present time. If we walk away from the stories that tell us we are intended to rule the earth (Which we don't have the knowledge or skills to do without terrible consequences like climate change, famine, disease outbreaks, etc) then I think it will be excellent for our collective mental health and outlook. Then maybe our stories and our religions would change, too.
I think it's bad because it causes a lot of judgement, and hatred towards other people just for believing in something different then what that person believes, it's terrible.
The middle east and India are good examples of why religion is bad for society. Instead of helping the people evolve and become 21st century they are stuck in a time loop of medieval thinking and practices because the religious dictators and religious leaders say that is how it is supposed to be.