>>24671916
The Holy Bible. The Thirty Years War killed 2.8 times as much of the German population as both World Wars combined. The Huguenot War in France killed 14 times as many as the devastating First World War. The First Crusade killed an astounding 66-80% of the belligerents on the winning side (by contrast, most soldiers on the Eastern Front survived the war) and depopulated the affected regions.

The Koran is a close second.