The reason pagan/tribal religions seem to have been less violent than modern theistic ones is because of the centralization of political power which corresponded to the rise of Christianity and Islam. Constantine didn't use Christianity to turn Rome into an engine of violence - he used the unprecedented concentration of power in Rome's state apparatus to use Christians as instruments of violence.
The Holy Roman Empire was a permutation of Roman central-statism, and it was the decentralized political environment of Medieval Europe that kept its expansion in check. Simultaneously, the Church kept the secular state powers from consolidating until the Protestant Reformation, at which point the nation state began to form in the vacuum.
I'm currently doing research for a book about this which will be called Pope and Pagan: A History of Church and State, from Nero to the Religious Right. It takes a sort of Hoppean view of European history, so I expect it will be very controversial amongst democracists. But it will probably be equally offensive to Christians of the Religious Right persuasion as well.