Professor Stevenson at the LSE, perhaps the best WWI historian in the world, has a quick note on why that war was “bad”.

The First World War was the first modern industrial war. Millions of shells were manufactured, and millions of troops were enlisted – meaning it was fought on a much larger scale than 19th century wars. It’s also an emblematic example of what is sometimes called a ‘bad war’ – one which didn’t really achieve its aims, where the casualties were far worse than expected and where the outcome was indecisive.