Religion is poor for a variety of reasons.
But the most important explanation why religion is evil, the reason that simply trumps all else, is that religious belief is bogus and corrupt by design.
To begin with, religious belief's animus is believing in made-up things regardless of proof (religious faith; belief without evidence), which is a profoundly false epistemology.
Second, it's focused on manipulating all kinds of cognitive prejudices to keep people believing in whatever religious traditions/doctrines have been built up on religious faith down some specific threads of cultural history, which is related to the first element.
This is why there are so many different religions, all arbitrary and contradictory, since they have no basis in fact and are just different threads of religious beliefs that have persisted over the centuries in different cultures and geographies (and which have now spread across the world as technology has eliminated physical barriers to contact by allowing us to communicate with everyone, anywhere in the world).
So, while there are a variety of other explanations why religion is evil, they are all secondary factors. The primary reason religion is bad is that religion - all religions,all religious practices - are incorrect, and not only incorrect, but also fictitious, having little bearing on truth (wrong) and replacing false beliefs that take people away from truth and away from coping with reality (bogus).
I may end my post there, but I'd like to answer at least one point that someone brought up. Cathedrals, of all things, he mentioned. And I said to myself, "Seriously?" What about cathedrals? Religious views are nonsense, but we wouldn't have majestic cathedrals if we didn't have faith. This is meant to be a religious statement. And I'm thinking to myself, "Anyone who uses that kind of logic obviously doesn't have a strong grip on fact (case in point), because, first and foremost, all you have to do is look around you to see great secular, not religious, feats of architecture and art." Trying to point out cathedrals is the epitome of selective ignorance, picking only a historical slice of a cultural slice in a world soaked in religion - indeed, in a culture where religion and the state were either one and the same, or extremely incestuous at best.
Another thing that came to mind was cathedrals. Is this for real? Like slaves to the State/God, erecting a structure to serve the Priesthood and false religious doctrines? Is that your situation? Where are the memorials to humanity's accomplishments and the common man's search for understanding of the universe? See my earlier mention of religion's corrupt existence, and this cathedral thing is a perfect example.
Religion is bad because of its false ideas and how they corrupt our thought and efforts in completely vacuous ways that have nothing to do with fact, and cathedrals are an example of this. Religion has kept us back and continues to do so because it is fundamentally flawed (see point number one, above). We would have placed humans on the moon at least half a millennium ago if humans were not so bizarrely enamored with cultural superstitions. Religion isn't just a hindrance; it's also a highly appealing drug. That is exactly why it has slowed human development so much.