The Crusades were terrible, but WWI and WWII killed far more people individually than all of the Crusades combined.
Crusades (1095-1291)
1,000,000-3,000,000 deaths.
WWI (1914-1918)
15,000,000-65,000,000 deaths.
WWII (1939-1945)
40,000,000-72,000,000 deaths.