It has been a major reason for war and cruelty in the world
War and cruelty are due to a lot of things, usually down to the profound arrogance and lack of empathy inside people at times.
In my observation, while there do appear to be wars caused by religion, in reality religion is more the topic than the cause.
The cause is the arrogance, lack of empathy, and cruelty itself in people, and the us vs them mentality.
When people blame religion for wars, it's almost as if they are saying the war would not have happened if it weren't for religion, and in their mind, it then becomes something where they think wars and cruel things wouldn't happen if it weren't for religion, and i think that's not quite right.
The us vs them mentality, the lack of empathy, the intolerance for differences, the will to power, the insistence on having and abusing power.
That is the problem.
Religious ideas and institutions sometimes facilitate cruelty and abuse of power.
Religious ideas and institutions can sometimes serve a protective role, though weak or inconsistent as churches don't have the kind of power and reach of governments usually unless they are in bed with governments, contaminating both.
But I think there is actually something naturalistic in people's brains that causes cruelty and war. Millions of years of evolution and competition and fighting to put one's own genes into the next generation have more to do with cruelty and war than religion per se.
Religion can serve as a vehicle of bad things, and it can also serve as a counterpoint and inspiration to resistance. It is rarely in and of itself the sole cause of things, good or bad, in my opinion.