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.
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.