Lieu: As for WWI, it may have been a terrible war in modern times, but it isn't the worst war or even the first worldwide war. It was nothing but another war between Europeans.
True, The Jws continually try to paint WW1 as the worst in history, and certainly a terrible time. But as Lieu says, it was essentially a European war, and was only extended to the rest of the world through the imperial colonies held by the protagonists. Even WW2 is misnamed, though it is more deserving of the tag, world wide war, than WW1.
The wikipedia gives a total for deaths as
The total number of military and civilian casualties in World War I was over 38 million: over 17 million deaths and 20 million wounded, ranking it among the deadliest conflicts in human history.
Though some give a higher number.
For sheer bloody butchery though, its hard to go past the mid-nineteenth century civil war that raged for something like a decade within China, as the Christian Taiping attempted to defeat the ruling Qing dynasty. A recent Chinese study places the death toll as fifty seven million.
A Muslim revolt occurred at the same time, and I'm unsure whether the above figure includes the death toll from that war in the southern (Yunnan) and western (Xinjiang and Gansu) provinces. If they are NOT included the death toll must be around 65 million.