The first world war was nothing, compared to the black death of 1347-51. The black death (bubonic plague) killed an estimated 34 million people in 4 years, 1/3 of the European population. In the following decades, millions more died, as the plague came back from time to time. In world war I, "only" 15 million people died. 9 million of these were soldiers on the battlefield. 6 million civilians looks like a lot, but many of these were russians killed in the revolution from 1917 and onwards, and in civil war in some European countries. All in all, the large cities were spared during WW1, it was a trench war, and the civilian population were in many areas unaffected (apart from food shortages). In world war II, 55 million died, half of which were civilians. Still, compared to the population size, the black death was a much larger catastrophy. But if I were to set some sort of "deadline" in the 20th century, the second world war would have to be it, not the first one.
Edite to add: Oh, and by the way, yes, people in 1349 believed in was judgement day coming, yes. In fact, many were sure it was the end of the world. And who would blame them.