We must first look at why violence occurs. I will start with the international level, and the largest form of violence—war. According to offensive realism, war occurs because states are always trying to increase their security. However, because international affairs is a zero-sum game, they can only achieve security by taking it from others. This is why war occurs over territory. Territory acquisition increases the power of a state, and therefore, its security. This logic also applies to war over resources—especially important resources such as Rare Earth Elements, which are used in the manufacturing of advanced weaponry. Now, I believe that these would be considered acts of secular violence, but what about holy war? Well, if you apply the same logic of the acquisition of land, it would seem that they are about power. It could also be that holy wars are about the power of the religion as a whole. By waging a holy war, by winning one, the religion is asserting its religious authority over others, and over the people it already has under its control. So, while there is a religious justification, holy wars (such as Crusades or Jihad) are actually secular wars for power.
Take, for example, the Inquisition. Some might say that the violence conducted by the Inquisition was without a secular cause—that it was a case of pure, unadulterated religious violence. I am not one of those people. Put yourself in the shoes of the Catholic Church, for a moment. Right now, you have dominion over all of Christendom (to some extent). Reading is not widespread and religious diversity is low. The best way to maintain power is to minimize free thought. If people think freely, then they can rebel against you. People who held heretical beliefs, therefore, must be removed so that your power will remain. So in that sense, the Inquisition (and all religious discrimination by a majority group against minority groups) are done for the sake of power.
Let us now turn our attention to religious terrorism—specifically that of radical Islamism. Now, one could say that they are fighting a war against “the infidel.” One could argue that terrorism by groups such as al Qaeda and Hamas is religious in nature—but I do not. If you look at the places where these terrorist groups originated—Iraq, Palestine, Afghanistan—they are all places that are still going through the process of colonialization. This is in itself a time marked by instability—without the added support of authoritarian regimes by Western powers—namely the United States. So while they may use religion as a justification, as a rationalization for their acts of violence, these terrorist groups are really acting as guerillas. They are fighting against oppression for the power of self-determination that has been denied to them by the West.