While by no means is it the top cause of war, like the picture suggests ,it definitely has directly or indirectly caused a very large amount of wars, and killed a large amount of people, the worst part of it is these wars are generally justified due to religious reasons. Nowadays religion doesn't cause as many wars as it used to and most religious conflict is based around extremist groups, but it is still a very prominent problem.
If you look throughout history, it comes down more basically to 'those people have those things that I think we should have'. Sure, it is dressed up in a number of ways, religion being one of them (educate the heathens so they will have a high quality of life like ours, right?), but it is generally just looking to expand the lands of the ruling power. The civil war, for instance, was about independence of course, but at the core it was 'the monarchy wants control of our land, and for us to pay them for having this land, but we want to have this land that we already battled our way through the countryside with the natives for'. It all comes down to wanting control of resources, stemming back to the age old equation of 'more resources = more survival'.
There are other aspects and motivators, of course. Looking to breed commonality of culture to avoid greater bloodshed in the future, same as when tribes would marry their kids to combine bloodlines and whatnot. Looking to spread religion for that common values and control base. Think about Constantine on his deathbed, mandating Christianity as the religion of his empire? It was to unite society under a common value structure so that the empire would have a standard of sorts, and they could depend on commonality of values to breed peace.
Very few wars are caused by religion. The main reason for war is politics and the corrupt and greedy hearts of humans. I'm not saying religion has not had its share in starting wars, but as much as the atheists LOVE to blame religion for all the wrongs in the world, religion is hardly the main cause. People are!
Sure religions hate each other, Catholics and protestants literally played cat and mouse for decades. But why in the any sort of benefit does killing an opposing religion do anything.
Crusades, best example, from west to east, it costs money to travel. Feed the troops, gather men, train, iron, wood, fabric, etc. this isn't like a few thousand dollars per soldier. Rather like millions of investment. Over several years. Its not like killing someone halfway across the world is a top priority while you couldn't promise a good enough harvest this year. Let alone the year before or the year after.
Crusades did happen, but they were not religious. The west gain back the lost knowledge since the fall of rome. It brought back new medicine, media, technology, trade routes, etc. they got rid of refugees, who otherwise had to be slaves, and even then did't have enough to feed them.
Sure you need some sort of poetic justice to motivate every other step, but the planning and execution is all financial. Or at least should be. Blind investments has resulted in failure as well but that is more due to nearsightedness rather than zealous fervor.