Religion truly is the opiate of the masses, and under its influence humans are capable of acts, thoughts, and deeds that they would otherwise not be capable of. That is where this thread is going, and while all of that may be true. It isn't really the heart of the matter. Religion foments divisions between people and peoples, and it is in those schisms between individuals and groups where evil festers.
Religion is the foundation from which racism, nationalism, feudalism, sexism, and slavery spring. It separates one group from another, and diminishes the respect of one individual for another. It doesn't just make it easier to justify bad acts against another. That is the natural byproduct of this way of thinking. Let us be clear about something religion in all its many forms is a way of thinking.
Religion isn't in itself bad, but it is bad in certain contexts. Ten thousand years ago religious thinking probably gave our ancestors a survival advantage, because it strengthened the group by giving it common purpose, and creating certain ways of behaving towards one another that minimized conflict, or allowed it to happen in a controlled form. However that was then, and this is now.
Religion puts up walls, and while those walls once helped us to keep our enemies at bay. Those walls have become our true enemy. The future of our species is becoming increasingly dependent on pluralism. A group of people who are sitting behind a wall going insane isn't self contained anymore. It doesn't really matter if it is just one group, and the other groups are pacifists. That one group can do damage on a global scale.
Religion by its very nature no matter what the adherent believes is a corrupting force. That actually does more harm now then it does good. If you doubt this at all. Look at the global trends. Nations where pluralism, non religiosity, and secularism are in the ascent. Very much dominate the Nations where that is not the case, and it isn't just in the form of militarism we are talking about.
Those nations having fewer boundaries between their citizens. Are able to work together to far greater effect. This is an evolution. Anywhere that religious thinking has started to wane. There has been a boon in the human capacity for creation. Everything goes right through the roof. Science, the humanities, civil engineering, and philanthropy. It is because religion is actually detrimental, and is that not a textbook definition of something that is evil.
Something that expressly harms the group. You can have any religion of any color you like, and have it lead by who ever you want, but it doesn't change the defining characteristics about religious thinking. It doesn't promote the exchange of ideas, or the creation of new ideas. Which are what drive progress.