What made world war 2 the bloodiest conflict in human history?

There are multiple answers to this question. Some ideas/theories include:

• The war's length
• it involved quite a few countries
• many people died not athrough direct violence but side effects of war (homlessness, destroyed food supply, ect.)
• the hollocaust is sometimes included to say this
• us nuking Japan

also as a side note the Aerican Civil war resulted in the largest loss of American lives.