Christianity isn't one united group. I would argue that most of the violence caused by religion was actually sectarian violence between groups within a single religion. For example most of the religious violence of Christians in the last millennium (excluding maybe the 20th century) was directed at other Christians. I think the same applies to Muslims.
Violence is a part of some peoples lives. They just need an excuse. There is violence within some families...doesn't mean that families by their very nature encourage violence or are responsible for it. Families are a unifying and essential unit...violence is just an aberration. Similarly with religions.
Religions have been responsible for a feeling of oneness among diverse groups across geography and other differences. The feeling of kinship among people within a religion is far more pervasive and far more powerful than any differences or violence among them.
Also, many of the modern humanistic values that civilized people (including atheists) promote has been taught and enforced by religions. These values are today an essential part of civilized societies because of the social control brought about by religions. The transition from tribal values to universal values of humanism, love, honesty, non-violence, charity, service above self and so on....have been possible largely because of religions.
Today many of these values could exist outside religions but they have been enforced and spread across the world largely by religions, over the centuries.