I know, but to have that evil thought supported by many to whom you relate, is dangerous.
In the same way any kind of extremism is dangerous, not just religious. If you hold a radical belief, and you're on your own, you're less likely to act upon it than you are if you have thousands of backers.
If you can then, somehow, justify it as right in the eyes of a supreme being then you're very much more likely to act upon it.
I appreciate these descriptions are more akin to radical / extremist actions but you can equate to everyday acts also. Naturally it's the evil acts which are dangerous, religion just seems to magnify things in the same way a very religious person with good intentions may go more all out in order to seek praise from their God.