In this post, I will explore the first reason, the way a religious person might interpret a random event as a "message" or a "sign" from their god.
Imagine this scenario. A man is walking across a beach that only he knows about and he sees a pattern that looks like letters in the rocky cliff nearby. He squints, and he sees a vague resemblance of the pattern to the sentence "Kill All Gays". Sure, the K in Kill looks a bit like an R, and the G is kind of lopsided and could easily be a crude unicorn if you turn your head jsut right, but you can still easily see the words "Kill all Gays" in the stones. He'd walked by here several times before, and never saw this here before.
Now, if this man were an atheist, then there is almost no chance at all that he would think it was special. He would realize that it was just a random pattern in the rock, he might note the humor in it, but he certainly wouldn't interpret as a sign from any god. After all, he has no god to interpret it as a message from, right?
Lets change our man to a Christian. Lets say he's a man who just the night before prayed to his god, saying "God, please, help give me some direction in my life. I hate my job, and I want some sort of goal to work towards." Lets also say that this man, while only a "moderate" Christian, is also a bit homophobic. Not a whole lot, just enough to feel somewhat uncomfortable around gay people.
So, this man walks across the beach and sees this pattern in the rocky cliff nearby, and how does he react? Generally, he will react the exact same way as the atheist. But there is a small possibility that this Christian (Subsitute Muslim, Jew, etc.) will see it and say "Oh, well maybe that's a message from god, a response to my prayer last night?"
Out of all the people (Very few, for sure) who will actually think this way, even fewer will actually act on this, but our man is just one of those people, and he goes to work the next day carrying a 5.56 mm ArmaLite Fully Automatic Assault Weapon and goes from office to office, killing everyone he knows are gay (He works in fasion, so that's a lot of people)
Obviously, not every religious person will act this way. However, this post is not meant to point out that all religious people act in a particular way, but that religious people are the only people that can act this way. Those few people who do act this way would probably not have had the drive to do so were it not for their religious faith.
"Religion is an insult to human dignity. With or without it you would have good people doing good things and evil people doing evil things. But for good people to do evil things, that takes religion." -Steven Weinberg
To be honest, religion is not the only thing through which a good person might do evil things. However, it is definately the easiest, quickest, and most effective way.
Another example compounding this effect is the interpretation of internal dialogue as a message from god. Normally, people talk to themselves inside their heads. This "internal dialogue" is integral to human reason: it is how we prepare our arguments in debate, how we advance our opinions beyond mere gut feelings.
In our new scenario, we have a man who is being constantly annoyed by the people in nearby cubicles at work, their boss is an asshole, and they hate their job.
One day, when thinking about all this, the man's internal dialogue says "Bring a gun to work and kill them."
The response of an evil person would be "Sounds like a good idea!" and he'd do it.
The response of a good person would be "Sounds like a horrid idea, no, I'm not going to do that!"
The response of a religious person would likely be either of those two, depending on if they were an evil or good person. But there is still the possibility of a person who the day before was praying for help at work from their god. This person might have the response "Okay, god" after which they go to work the next day carrying a 5.56 mm ArmaLite Fully Automatic Assault Weapon and go from office to office, killing everyone that annoyed them.
As I have said before, I know very well that most people would not do this. However, the only way a person can interpret this as a message from god is if they are religious. It is essentially impossible for a full-blown atheist to have this reaction.
Another way this problem occurs is in cases of "Exorcism". Some well-meaning priest is brought a young boy with a mental disorder, and they perform an exorcism. These rituals can cause bodily harm, and also leave the victim unhelped (Unless it is a case where the person beleives they are possessed, in which case botht he problem and the solution are placebo effects.)
"Anneliese Michel, a 23-year-old who had her first psychiatric episode at the age of 16... apparently suffered from depression, epileptic seizures, and various hallucinations. Her zealous Catholic family believed she was possessed by Satan and recruited two priests who performed the exorcism ritual 67 times on the mentally ill woman. At the time of her death Anneliese weighed about 69 pounds (31kg). The priests and her parents received only suspended jail sentences, but the Church began requiring priests to obtain permits to perform exorcisms, and the permits are not easy to get. "
-The Skeptics Dictionary
Could this have happened in any other case but for religion?
"Oh, that's uncommon, though" you may say.
It's not as uncommon as you think.
"
Pentecostal ministers in San Francisco pummeled a woman to death in 1995, as they tried to drive out her demons.
In 1997, a Korean Christian woman was stomped to death in Glendale, Calif., and in the Bronx section of New York City, a 5-year-old girl died after being forced to swallow a mixture containing ammonia and vinegar and having her mouth taped shut.
In 1998, a 17-year-old girl in Sayville, N.Y., was suffocated by her mother with a plastic bag, in an effort to destroy a demon inside her.
In 2001, a 37-year old woman, Joanna Lee, was strangled to death in an exorcism by a Korean church minister working in New Zealand. The minister, Luke Lee, was found guilty of manslaughter."
-The Skeptics Dictionary
I could find more examples, but I don't feel like taking the time unless I am asked to. The point is, there are many evils done by well-intentioned people that can only be done in the name of religion.