Well, there was the time that the Krikkit decided to wage war on the entire universe for 2000 years - I believe the estimate was about two grillion casualties.

In case you’re being serious, Douglas Adams was an author of comedic fiction and the babel fish is from his Hitchiker’s Guide to the Galaxy book and allowed different races from different planets to communicate with each other.

Trying to compare real world history with fictional possibilities is non-sensical.

You might like https://en.wikipedia.org/wiki/List_of_wars_by_death_toll

Though you might want to look at your wars by death toll as percentage of total population?

No, but Second Sino-Japanese War, Taiping Rebellion and Transition from Ming to Qing are comparable.

WWII wasn't the bloodiest conflict when you adjust for world population at the time. E.g. the Mongol Conquests or An Lushan Rebellion probably killed more.

In terms of body count, those still win, because Mutually Assured Destruction keeps giving people pause. But the Internet has turned into a machine for goading people into making a mistake, and it will only take one.

Meantime, thus far it's used just for local genocides, like Myanmar.