I've read the generalization that all religions can be misused to support violence, and the sweeping accusation that all religions have in fact been used to support atrocities and violence. I think, however, that this is not technically correct. My understanding is that there are several religions for which nonviolence and anti-warfare principles are central to the core belief system, and I am unaware that these religions or sects have perpetuated any atrocities, violent persecutions, wars, or generally supported aggressive warfare or communal violence. I would include in this list:
I think it's also important to recognize that when religions have become militant, or supported nationalistic or imperialistic wars, there have been heroic persons who stood up for a vision of peace and humanistic spirituality that rejected warfare and nationalism.
Let's start with Buddhism, which has recently been in the news because of Buddhist anti-Muslim and anti-Christian violence in Myanmar and Thailand (plus anti-Hindu violence in Sri Lanka). When anti-religious zealots claim all religious are violent, people sometimes suggest that Buddhism isn't so violent, and the example to contradict this claim is the fact that Japanese Buddhist leaders generally supported the nationalist militaristic agenda of Imperial Japan during World War II. However, a few Buddhists disagreed with the nationalism and militarism, and suffered for their thought crimes; people should know about figures such as Makiguchi Tsunesaburo (1871-1944) and Toda Josei (1900-58), or other Japanese who opposed militarism and nationalism. Within Buddhism, there have been peace-oriented benevolent societies trying to improve people's lives (e.g., Soka Gakkai International; Buddhist Compassion Relief Tzu-Chi Foundation), although like any large human institutions, these groups are plagued by internal politics and intrigues, the corrupting influence of power, authoritarianism, and the inevitable problem of human personalities importing their own mental and cultural issues into their religious organizations. This is not a problem unique to religious groups or religion: it is a problem of human nature and any large organization using hierarchical structures and bureaucratic control systems (which seem necessary in all large human institutions). Some Buddhist activists for peace include members of the Buddhist Peace Fellowship, Thích Nhất Hạnh, (of Vietnam, now in France) and Daewon Ki (of Korea, now in Hawaii).
Lately, some people have claimed that Islam is especially violent. In the Islamic world, there have been a number of famous champions of peace, including Shaykh Aḥmadu Bàmba Mbàkke (1853–1927) in Senegal, Khan Abdul Ghaffar Khan (1890-1988) in British India, and Mirza Ghulam Ahmad (1835-1908) also in British India (although Ahmad's movement in Islam, the Ahmadiyya Community, may or may not be an independent non-Islamic religion, depending upon your premises of how religions and religious movements should be defined and distinguished). Currently there are many significant Muslims working for peace and non-violence, including such distinguished persons as the scholars Farid Esack (South Africa) and Sheikh Abdallah bin Bayyah (originally from Mauritania), as well as the Palestinian activist Mubarak Awad; and there are many non-violent movements in the Muslim world, including supporters of the Green Movement in Iran, The Afghan Youth Peace Volunteers, La'Onf in Iraq, and so forth.
There are groups like the World Council of Religions for Peace as well.
The Christian tradition of non-violence is probably well-known (Martin Luther King, Jr., Leo Tolstoy, etc.), as is the Hindu tradition (Mahatma Gandhi).
People who have supported violence, warfare, and oppression of other human beings have often justified their murderous aggression and heartless cruelty by appealing to religious traditions or values, but there are some religious that have not been used for violence, because their core teachings are for peace and against war. Likewise, religious have inspired leaders who worked for peace and human dignity. Secularists and atheists have also worked for peace, and have also used non-religious secular ideologies to justify cruelty, violence, and warfare. I'm very skeptical of arguments that religions are especially bad ideologies, or that secular non-religious values are inherently superior or less likely to be used to justify oppression, cruelty, and warfare. It seems to me that religions offer an important alternative view to whatever ideology or value system is supported by the state, the ruling elites, or political leaderships.