It’s hard to imagine the kill rates in WW I. Our own War between the States was unnecessarily deadly because Napoleonic tactics were used with mid-19th Century weapons. WW I was a case of Napoleonic tactics used with 20th Century weapons.
WWI, I think, was MUCH more a conflict of not just tactics, but technologies of totally different types.
In the Am Civil War, they could’ve used repeating rifles but just kept on with single-shot percussion muskets. That’s just an example.
Not that I’m any expert, just a casual observer. ;-)