You're suggesting that religion was a big help in, for example, the Native American genocide?
Among other things.
For one thing it takes effort to intentionally skew religious views to suite ones actions, if those actions seem incongruent with religious values. So in that sense religion would be a hindrance.
Well the Crusades disprove your point, for instance, so do deaths that can be directly attributed to religious fanaticism due to literal interpretation of some passages in holy books, and so on and so forth.
Once you find proper argumentation inside a book seen as holy, you attribute such actions to the mandate of a divine source and they become morally beyond debate. Whatever atrocity committed in the name of "insert name", becomes morally righteous. This is particularly true in the religions that hold truths by revelation, as Abrahamic religions. It's a world of trouble to change a single line within their canon.
Plus, the barriers of superiority/ inferiority fomented by religions (believers vs non- believers) are pasture for all sort of corruptions.
Religion is clearly not the only catalyst of total war and other forms of indiscriminate violence. Nobody here is claiming that to be the case as we are all aware that people seem to be able to invent all sorts of rationales for mass killing without feeling the need to cite the will of God. However, religious violence can take a particularly intense and ruthless character, if the objects of that violence are seen as blaspheming or insulting God, as the enemies of God or his people, as savages, heathens, without soul and what have you. My point is that although religions aren't the sole rational used to justify violence, some of them have violence so deeply rooted in their scriptures and traditions that under the right circumstances they become facilitators instead of hindrances, as you claim, to wage war upon those who don't share the same faith.
But the most absurd part of this is that religious or even minor ideological motivations were needed at all. The American continent was a huge resource just waiting to be taken. If those who took it had no religious or ideological motivations or justifications for taking it would they have been like, "damn, look at this place, all this land, the gold... but we can't take it because we have no religious or ideological basis for doing so. Back to the old world, boys!"
Absurd is your denial to acknowledge the tremendous power religion can over the masses, how it has been used in history to shape public opinion and action and the way in which it can make people's ideas malleable to pursue the worldly purposes of an elite - religiously motivated or not - thinking their actions are ethical.
Guns don't kill people. People kill people. Religion doesn't kill people. People kill people. That's what you are saying. We all know where that leads.
Malcom though, is providing a deeper analysis of this issue. If I would settle for the removal of hate speech from religious books, he analyses the belief system itself and how it influences people's minds due to the way it is conceived. He provides an explanation on why -and how to a certain extent- as species, we should go beyond religion and justifies it. Somehow I believe we're centuries behind such step, but it's a beautiful dream.