How has Christianity managed to be such a violent religion even while its adherents have so often promoted it as a religion of peace? Why has Christianity been a source of violence between Christians as well as violence directed against non-Christians? Justifying violence and war hasn't been very hard, unfortunately, and we can identify some good examples of that by looking at Crusades.
Christian Justifications for Violence
The Crusades aren't the only example of violence in Christian history, but more than any other era it was characterized by mass, organized violence that was explicitly justified with specifically Christian arguments. The ability of Christian leaders to launch multiple crusading wars against non-Christians and Christian heretics will tell us a lot about the relationship between Christianity and violence.
In The Crusades: A History; Second Edition, Jonathan Riley-Smith writes:
For most of the last two thousand years Christian justifications of violence have rested on two premises.
The first was that violence — defined crudely as as an act of physical force which threatens, deliberately or as a side-effect, homicide or injury to the human body — was not intrinsically evil. It was morally neutral until qualified by the intention of the perpetrator. If his intention was altruistic, like that of a surgeon who, even against wishes of his patient, amputated a limb — a measure which for most of history endangered the patient's life — then the violence could be regarded as being positively good.
The second premise was that Christ's wishes for mankind were associated with a political system or course of political events in this world. For the crusaders his intentions were embodied in a political conception, the Christian Republic, a single, universal, transcendental state ruled by him, whose agents on earth were popes, bishops, emperors and kings. A personal commitment to its defence was believed to be a moral imperative for those qualified to fight.
Religious and Non-Religious Justifications for Violence
Unfortunately, it's common to see people trying to excuse religious violence by insisting that it is "really" about politics, land, resources, etc. It's true that other factors usually exist, but the mere presence of resources or politics as a factor doesn't mean that religion is no longer involved — nor that religion isn't a cause or being used as a justification for the violence. It certainly doesn't mean that religion is being misused or abused.
I don't think that there are any religions where religious doctrines have not been brought in the service of justifying war and violence. I also don't think that these religious doctrines were merely used as convenient "excuses" which people used for doing what they wanted to do in the first place. Perhaps this has been the case in some situations, but for the most part I think that people have genuinely and sincerely believed that war and violence were logical outcomes of their religions.
Religion and Complexity
It's true that Christianity makes a lot of statements on behalf of peace and love. Christian scripture — the New Testament — has a lot more about peace and love than about war and violence and little that is attributed to Jesus really advocates violence. So there is justification for thinking that Christianity should be more peaceful — maybe not perfectly peaceful, but certainly not as bloody and violent as Christian history has been.
Nevertheless, the fact that Christianity offers many statements on behalf of peace, love, and non-violence doesn't mean that it must necessarily be peaceful and that any violence committed on its behalf is an aberration or somehow un-Christians.. Religions offer contradictory statements on all issues, allowing people to find justification for just about any position within any religious tradition of sufficient complexity and age.
Religious belief systems are like any complex ideology or philosophy — which is to say that they don't force only certain conclusions and no others. When it comes to non-religious belief systems, people have come to terms with this. They may not like it very much, but they accept that differences and disagreements are necessary. Religions, though, don't typically allow for this and so we have people fighting over the One True Interpretation of religious tradition. This, ironically enough, merely serves to add to the violence.