How can we define "atheism"?  The term is a bit ambiguous.  Some atheists might say that they reject all possibility of a deity or deities.  However, the problem with this position is that the existence of deities is not scientifically falsifiable.  In other words, we can not disprove a god or gods any more than we can disprove fairies or the ubiquitous Flying Spaghetti Monster.  Therefore, I propose that atheism should be defined in the most inclusive sense, i.e., as "the absence of belief in a god or gods."
        Is absence of belief an ideology?  Christian apologists will sometimes claim that not believing in God amounts to a belief system, and even go so far as to say that atheism may lead to immoral behavior.  But this is a logical non sequitur.  There are perhaps millions of gods whose names I have never heard of and whose worship I am not familiar with.  Does my absence of belief in all the countless gods that people have worshiped over the millennia motivate me to do do good or evil?  Of course not.  That would be absurd.  When atheism is attached to a political ideology like communism, it is the political ideology that motivates people, not atheism. Lack of belief does not impel people.  Therefore, how could lack of belief impel people to do evil?  Blind obedience to a supposedly infallible authority, however, can lead good people to do morally unconscionable things. That's what Professor Steven Weinberg meant when he said the following of religion:
       
        "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."  (Address at the Conference on Cosmic Design, American Association for the Advancement of Science, Washington, D.C., April 1999)
       
        A good example of religion leading to the morally abhorrent can be found in the biblical story of Abraham being commanded by God to kill Isaac.  Abraham could not have wished to kill his own child, but he was willing to defer to God's authority and do the evil deed.  I don't care if Zeus or Jehovah or Ganesh were to fly down from Space and order me to kill my child.  I'd tell any entity evil enough to request such an abominable act to go spend some quality time in its own hell.  
       Ah, but God graciously stayed Abraham's hand at the last moment.  Never mind the mercurial, sadistic, insecure, narcissistic nature of a god that would ask a man to kill his own son in the first place.
       
        "now I know you fear God"  (Genesis 22:12)  
       
        Why is God so insecure?  This is especially bizarre since God is supposedly all-knowing and all-powerful.  Which brings me to the next point.  
        Why does God supposedly create people with free will and then punish them horribly if they exercise that free will and choose to believe differently?  That's not a choice.  It's God's way or get hacked limb from limb.  All powerful Jehovah chooses a small band of illiterate desert folk to be his people, then gets jealous about the people in neighboring lands worshiping other gods.  He says thou shalt not kill (people within the tribe of Israel), but then tells the Israelites to go kill other peoples who are worshiping competing gods.  Jehovah the all-powerful needs the Israelites to do his bloody work for him--he just doesn't seem to have the power on hand to do it himself, and so commands the Israelites to go exterminate enemies such as the Canaanites and the Amalekites down to the last child:
       
        "Now go, attack the Amalekites and totally destroy everything that belongs to them. Do not spare them; put to death men and women, children and infants, cattle and sheep, camels and donkeys."  (1 Samuel 15:3)

        It takes religion to make people do wicked things they would otherwise hesitate to do.  There is some historical evidence that even in ancient times, soldiers would hesitate to kill children. Athenians would kill all the adult men of a rebellious colony, but they would usually spare the children.  Human beings do have some natural compassion for children. We cringe at the idea of children suffering. How often have we, without thinking, found ourselves rushing to help a child who has fallen down or injured herself?  It is often referred to as  the "maternal instinct," but it exists in men too.  The biblical Israelites nevertheless slaughtered the children of the Canaanites and the Amalekites in accordance with their god's command.   Here is Christian Apologist William Lane Craig's defense of Israel's genocide of the Canaanites:
       
        "Then Craig said: 'But, how could God command that the children be killed, as they are innocent?  I would say that God has the right to give and take life as he sees fit. Children die all the time! If you believe in the salvation, as I do, of children, who die, what that meant is that the death of these children meant their salvation. People look at this [genocide] and think life ends at the grave but in fact this was the salvation of these children, who were far better dead…than being raised in this Canaanite culture.'"
(James Rothwell, "Craig Strikes Back at Genocide Smear," The Oxford Student, 27 Oct. 2011)

       If you think that's a reasonable defense, then you might want to deeply consider what it is you really value.

        Some Christian apologists try to draw attention away from religion by citing examples of situations where people unquestioningly committed atrocities in the name of an ideology rather than a formal religion.  My reply is as follows:
      
        Whether predicated on blind faith in a god, or a semi-divine dictator, any blind faith in an authority that is seen as infallible, any unquestioning faith in a man or men that are seen as more than just human, is religious.
       
        Religious faith can sometimes make "good" people do "bad" things because it calls upon us to suspend critical reasoning and simply trust in a higher authority.  Stalin was a little more than just human to his people and took advantage of a populace that had been taught to take things on faith and trust unquestioningly in the divine authority of the Russian orthodox church for hundreds of years. As Bertrand Russell, the eminent logician, once said,
       
        "As soon as we abandon our own reason, and are content to rely upon authority, there is no end to our troubles."  (Bertrand Russell, An Outline of Intellectual Rubbish)
       
        Stalin was an atheist, but he took advantage of millions of credulous peasants who had been subject to the saint worship of the Eastern Orthodox church for hundreds of years.  Stalin saw an opportunity to take advantage of all of that blind faith in the populace and effectively replace the church as the new divine authority.
        And what of the ideologies of criminal cults?  Charles Manson's followers were inspired by their messianic leader to commit murder.  I submit that the Manson "family" was religious.  Manson's followers undeniably saw him as more than just a man. One has only to read the interviews with former members of his cult.  He was starting his own religion, and his followers had religious fervor for him, just like the religious fervor of the Christians who carved the flesh from Hypatia's bones in fourth century Alexandria, back when Christianity was arguably still a cult, albeit a strong and growing one.  Every major theistic religion today started as a cult, i.e., a new religious movement with beliefs considered unorthodox at the time of its founding.  
        Hypatia was a woman and a philosopher of the Hellenistic tradition who tried to prevent Christians from defacing or altering the scrolls in the library of Alexandria, among the scrolls were the works of Archimedes, Aristotle, and Euclid.  She was murdered by a mob of Christian zealots in the year 415 AD.  The following is an account by Socrates Scholasticus, which was written about 450 AD:
      
        "For as she had frequent interviews with Orestes, it was calumniously reported among the Christian populace, that it was she who prevented Orestes from being reconciled to the bishop. Some of them therefore, hurried away by a fierce and bigoted zeal, whose ringleader was a reader named Peter, waylaid her returning home, and dragging her from her carriage, they took her to the church called Caesareum, where they completely stripped her, and then murdered her with tiles [oyster shells]. After tearing her body in pieces, they took her mangled limbs to a place called Cinaron, and there burnt them."  (Scholasticus, The Historia Ecclesiastica VII.15)

        Christians have asked me whether the people who killed in the name of Christ in the past were following Christ's teachings.  I can think of a better question:  if the Church had not elected to establish Christ as divine, if Christ had been seen as a philosopher and a social reformer, and not as a god, would thousands have been killed in his name?  Did Socrates inspire people to kill in his name? No, of course not.  It was the idea of Christ's divinity and the supernatural aspects attached to him that inspired religious wars.  Allow me to clarify why this should be.
        If one believes that Christ is the one true god, and if one truly believes this life is but a brief prelude to an infinity of Heaven or Hell, then by comparison, this life has almost no value.  The afterlife becomes the real priority.  We need give no thought for the morrow.  Those who killed in the name of Christ may have felt completely justified in doing so in the interest of saving souls from eternal damnation.

        “Do not think that I have come to bring peace to the earth. I have not come to bring peace, but a sword."  (Mathew 10:34)

        For many conquistadors, missionaries, inquisitors, and crusaders, the end may have justified the means.  Better to kill the heathens than to allow them to continue living in sin.  Better to break the body of the heretic or witch on the rack than for the heretic's immortal soul to suffer for eternity in the lake of fire. Far better that children go to an early grave, sent posthaste by the swords of the righteous than for those children to live in a heathen culture.