@Unbeliever: It seems to me that you think only people who follow a specific faith are violent, that cannot be true, religion teaches self-control, it teaches a person to live his life within defined boundaries.
Most people here were arguing that religion, all of the religions, are counter-productive and lead to violence and have lead to violence over millenia.
Nonetheless, some religions have a far greater tendency for violence owing to their dogmas. Hinduism is an established, religion, the oldest of the major religions, but due to the lack of universalit ambition it was never as threatening or violent as the two major Abrahamic religions, Christianity and Islam. Religions have changed the dogmas, created now dogmas and have evolved with time. Nonetheless, Hinduism has evolved over the past few centuries to create violent dogmas something it never had. Regardless of this, universalist ambitions among major religions can be found only (in a big generalized sense) among Islam and Christianity only. Orthodox Judaism prides itself on its royalty and Hinduism was never universalist. Buddhism is far from seeking domination and perhaps can't even be brought into the same group as the other ones for it's more like a spiritualist path than organized religion. Taoism and Shintoism never wanted to and do not want to convert the whole world to their beliefs, they don't foresee a inter-faith ultimate battle before judgment day nor do they label believers of other faith as inferior citizens or anything like the Abrahmic religions.
Besides Scientology (defined as a cult in France though), all post Judaism religions have relied on revitalizing the old beliefs, bringing in new books, scriptures and ideas but have essentially said that the previous thing wasn't complete and I'm here to complete it for you (Mormonism for Christianity, Ahmediyya, Ismailism and others for Islam and other numerous examples). It is difficult today to outright deny previous religious figures hence it is always I'm here to complete the word for you (remember Gohar Shahi, Yousaf Ali and others known as False Prophets).
Zeus, Thor, Horus and Poseidon are dead and we regard Mormonism with its healing underwears to be funny, perhaps our thousandth generation will laugh at our beliefs. But the entrenchment of religion (of the Judo-Abrahmic) kind has been very efficient and owing to the fact that they borrow a similar lineage and history (explanations will be different) and they are far better prepared than the Greek, Aztec or Egyptian Gods. Written accounts and unifying books have been a major source of their strength as well, as previous religions existed in a time when oral traditions were the main source, could be corrupted easily and weren't as powerful besides most of them never invoked the presence of a prophet on the ground.
To abstain from that which is wrong and to move towards what is right. Might I remind you that less than 4% of the total population of planet earth is Atheist while the remaining 96% believe in God in one form or another, however only a few hundred thousands are religious extremists and even fewer, possibly a few thousand are terrorists
The lack of atheism does not suggest that religion is somehow right. The figure might have stood at 0.004% in the 1600s. Increasing population suggests a far better understanding of the world has lead people to believe that religion, in all its forms, is a human creation and dis-belief is necessary. Scientific advancement has strengthened the case.
Man needs the idea of God to thrive, It is this idea that gives him strength when he has none, gives him hope when all is lost, gives him the promise of justice to whom injustice has been done.
There is no recorded evidence of a Judo-Abriahmic prophet arriving in China. China has a very well recorded history and the lack of such a prophet suggests that there never was one in that region.
Nonetheless, Confucianism created a very peaceful community that did not need a deity to pray to, to rely on and was based on what we can call today, Secular Humanism.
If religion makes one happy, then so be it. Everybody gets his share of mental peace from different sources but as I put it later, religion becomes an ideology rather than a personal faith and hence is a divisive belief. A personal belief is something everybody can respect.
Morality without Religion is one of the earliest and best discussed topics in Philosophy. But if you want a practical example then besides their socio-economic well being, the Scandinavians who are highly atheistic and irreligious are the biggest aid donors (as a %age of GNP) and are a very peaceful and law abiding society.
You may get rid of this ideology but how many other factors that incite people to kill will you try to do away with ? IRA killed for freedom, not for religion, Hitler killed for white supremacy, not for religion, Ted Bundy killed for reasons he could not explain himself but it definitely wasn't religion.
Nationalism and Religion have killed a lot of people, but religion wins the game of deaths. Nationalist ideologies will lead to violence, always. Similarly, Nazism and Bolshevism lead to millions dying but they weren't seeking an end to religion rather seeking dangerous political ideologies.
Serial Killers are a whole different ballgame. Criminal Psychology is a very interesting field.
Atheists are just as prone to exhibiting violence as any Muslim, Christian, Jew or Hindu.
Atheists might and have killed (Stalin) for political goals but will not kill for religion. As history has shown us, religion is the most divisive of all ideologies and the most brutal one as well. There might never have been any crusades (although other wars would have taken place), no forced conversions across Europe, no conquers in the name of religion and no inter-faith fighting at all.
A world without religion does not mean an entirely peaceful one but it sure means that there'll be better harmony and greater peace than there is. Geo-political ambitions will involve violence and war but religious glorification of war is a very big incentive that will no longer be there, hence populace won't be easily convinced as they are today by using divine rights and using religious dogmas.
The New Age Atheists (Dawkins, Hitchens, Harris, etc) aren't just atheists (although all atheists disagree with this very word for it implies that they are denying something while there isn't any hence the word dis-belief) but they are anti-religion and anti-theism for in their and many people's thoughts, religion is counter productive and leads to delusions and violence and hence it needs to be eliminated rather than being left to expand. In their debates and writing, they more than often become classic western imperialists and arrogant (especially Harris) which is reprehensible.
Many people are okay with others keeping their faith personal and not throwing that into each others face, but the sad reality is that religion is not a personal faith rather seeks expansion (especially in the case of Christianity and Islam) and irreligious people thus have no option than to call for the elimination all religious thought. Just like blasphemous speech is offensive to religious people, proclamation of a divine being (Thank God, May God ..., etc sentences) offend the non-religious people. Only in a dream world can faith be personal, and that is something perhaps hardly anybody would complain against. But the sense of pride and proclamation that is necessary with Judo-Abrahmic religions (and the pride associated with Islam especially) is disturbing to most atheists/people of other faiths. A friend of mine always says that he'll be perfectly fine with a guy praying to a dust bin, he has no problems with it as along as he keeps it to himself (including praying in public) but the sad reality is that tomorrow he'll set out on a proselytizing mission and start converting others to his dust-bin religion and tomorrow start crying war on other people. Also, as goes with most religious people, they do not want people of other beliefs to comment on their beliefs but that is a very arrogant and failed statement to make. Faith isn't the monopoly of one group or person and everybody has the right to comment (in a scholarly way and not bad mouthing) on their beliefs.
Like Daniel Dennet says, Yes everybody would like that there should be a God who should reward people for their good deeds and punish bad people for their deeds because the world is not a just place, but this does not mean that one has to believe in a non-temporal, non-spatial omnipotent, omnipresent and omniscient divine being.
Didn't want to discuss religious thought today or any day here but thought it would be necessary to explain some ideas that you would have failed to see.
PS: As old China came into the debate, it reminds me of the fact that same-sex marriages used to take place in ancient China as there wasn't any divine sanction to kill homosexuals back then.