How destructive/deadly was World War I? What were the major reasons why it was so deadly?

The casualties of WWI is around 40 million. They had many new technologies (u-boats, poison gas, chemical warfare, submarines etc) caused many deaths. The soldiers stayed in dugout trenches, many of them got sick from the living conditions or were malnutrient because they didn't have much food.

Because of the weapons they used, weapons like heavy artillery, poison gas and machine gun.