While generally the flu and even pandemics more broadly affect mostly the very old and very young, Spanish flu killed people almost indiscriminately. A total of 50 million people died of Spanish flu, and about 500 million people were infected - a third of the world's population at the time. It decreased average life expectancies by, on average, 12 years. However, what was unusual about these deaths was that many 20-40-year-olds were killed, while populations of elderly people were virtually unaffected. This is extremely unusual for an influenza pandemic. It is theorised that this may be because elder people lived through the Russian flu pandemic at the end of the 19th century and were therefore immune.
[P1] Spanish influenza was unusual in that it was particularly fatal to younger people. [P2] This makes it a much scarier, and therefore much worse, pandemic.