History has recorded some truly terrible events. Perhaps the worst of them are the mass murders orchestrated under the regimes of Stalin, Mao, and Hitler. Which has resulted in a oft cited proclamation that atheists have more blood on their hands than all other religions combined.
To begin with, atheism is not a religion. It is the absence of a specific belief, the belief in the existence of a God or gods. That's it. That's all it means. In order to build a philosophy or worldview from that point one needs to add something. Humanism, naturalism, or perhaps one of the classic philosophies like Stoicism would be necessary to then provide some kind of guidance/framework for behavior. Which are themselves not religions either, though they do act as substitutes to some aspects of religion in ways that atheism alone simply cannot.
Stalinist and Maoist communism became such a substitute philosophy, imposed from the top down upon the Soviet and Chinese peoples. That the architects of those atrocities happened to be atheists is not actually relevant, because it cannot be shown that it was their atheism that motivated them to commit mass murder. Saying that the great purges of the 20th century communists were caused by the atheism of Stalin or Mao is no better supported than saying they did so because of their shared hair color. Stalin and Mao both had dark hair (before age caused greying and/or balding), so are we then free to conclude that it was their hair color that inspired their purges? No, not without evidence to support such a statement.
Instead their purges were conducted under a known set of justifications and paranoias that revolved around the individual mental unhealth of those men and their Soviet/Maoist philosophies. These were deeply irrational men who employed/empowered psychopaths against their own people to horrific effect. It ought to serve as a warning to us all that irrational beliefs and people are profoundly dangerous.
Moreover it can be shown that the presence of religious beliefs does not prevent similar inhumanity. One need look no further than Hitler and Nazi Germany. Adolph Hitler was a Roman Catholic. He was not an atheist and enacted policies that persecuted atheists. The overwhelming majority of the Nazi Party officials that carried out the Holocaust were Lutheran or Roman Catholic. Not only did their Christianity not prevent them from committing genocide, it served as a twisted justification for doing so. No, Nazi atrocities cannot be blamed on atheism either.
"I am now as before a Catholic and will always remain so"
-Adolph Hitler, to Gen. Gerhard Engel, 1941
In the Spring of 1933, Hitler outlawed atheist and freethinking groups in Germany.
BERLIN, May 13. - In Freethinkers Hall, which before the Nazi resurgence was the national headquarters of the German Freethinkers League, the Berlin Protestant church authorities have opened a bureau for advice to the public in church matters. Its chief object is to win back former churchgoers and assist those who have not previously belonged to any religious congregation in obtaining church membership.
The German Freethinkers League, which was swept away by the national revolution, was the largest of such organizations in Germany. It had about 500,000 members ..."
-New York Times, May 14, 1933
"The greatness of every mighty organization embodying an idea in this world lies in the religious fanaticism and intolerance with which, fanatically convinced of its own right, it intolerantly imposes its will against all others."
-Adolf Hitler, "Mein Kampf" Vol. 1 Chapter 12
"The National Government will regard it as its first and foremost duty to revive in the nation the spirit of unity and cooperation. It will preserve and defend those basic principles on which our nation has been built. It regards Christianity as the foundation of our national morality, and the family as the basis of national life...."
-Adolf Hitler, Berlin, February 1, 1933
"In the ranks of the movement, the most devout Protestant could sit beside the most devout Catholic, without coming into the slightest conflict with his religious convictions. The mighty common struggle which both carried on against the destroyer of Aryan humanity had, on the contrary, taught them mutually to respect and esteem one another."
Do you consider the value of a human life to be greater than the value of an animal life? I was in an online conversation about this topic not long ago in which the relative values of various kinds of life were debated. Though I argued that my personal ethics placed the value of human life way above that of animals, that did not mean that I thought we ought to mistreat animals or that sadistic cruelty ought to be tolerated in our societies.
I personally feel a responsibility for the animals under our care, to provide for them a life that recognizes suffering and seeks to limit it. Likewise with wild animals, I think we should take steps to prevent human caused extinction. But that does not mean that I would place a greater or even equal value on an animal's life as compared to a human's under normal circumstances.
In a general sense, the value of an animal's life, its right to life, is a human concept not one derived from Nature. What right to life has the zebra being eaten alive by a clan of hyenas? None. Nature is harsh, cruel, and unfeeling, if you must personify it. Concepts of rights come from us, from our desire to be humane, which I applaud. But those human origins of the right to humane treatment represent values that flow one way, from us to them. We have no right to expect similarly compassionate treatment from the rest of the living creatures on Earth.
That we consider such things and make our moral choices based on such things makes us the responsible party. I think it makes us obligated to some extent. But that general obligation in no way implies a moral equivalency to humanity between the value of a human life versus an animal life. Nor should it.