If you think about it, evolution has led us to a situation where we need to overcome our natural urges to "tribalize." Religion and politics and race and many other factors all contribute to polarization, which is the real reason we have so many conflicts.
Overpopulation merely leads us to the situation where polarized tribes (or religious groups or political groups or take your pick...) come up against each other for resources (food, living area, energy, water, take your pick). However, that conflict could occur because of overpopulating the planet or the world or a single island. It is our inability to live together in peace without first having someone beat us over the head with a heavy, blunt object that is our built-in downfall.
Religion tries - and fails miserably - to beat us over the head with the "Hell and Damnation" stick so that we live together. The religions fail because of the multiplicity of religions that have different world views, some of which could never be reconciled. But this in turn occurs because religion is man-made, not divine. (If religion were truly a creation of divinity, there would be only one and its veracity couldn't be questioned.) For this reason, I don't hold religion as having any more blame than anything else.
It will always come back down to Man's stubborn nature and his reluctance to accept things that are different to his own viewpoints. That is, of course, a survival trait in a sparsely populated world - but not in a crowded one.