Why Knowledge Is Power
If you want to destroy humanity, burn all their books**.
Humans differ from all the other animal species on Earth in one very distinct way, we build upon and pass down our knowledge from generation to generation. We started with simple stone tools and were able to eventually forge iron weapons, build steam engines, and cure previously incurable diseases. Knowledge is humanity's greatest weapon, but at the same time, it is also our greatest weakness.
After building so much knowledge, we've come to rely on them rather heavily. What do you think will happen if all our books and accumulated knowledge were to disappear one day? Imagine that they really disappeared, what will happen to a city if there was a power outage and no one was able to fix it because they don't have the manual or schematics? And what if doctors can't treat certain illnesses anymore because no one remembered how and all the reference books and research papers are all gone?
If humanity were to start from scratch, will we be able to survive? How many people do you think knows how to start a fire, or build basic hunting or farming tools when we are so used to having food delivered to us? How many people do you think can identify and avoid poisonous plants and insects without a reference book, or perform basic first aid without a first aid kit?
Unlike other animal species that rely on their innate abilities to fight and fend for themselves, humans have long evolved to rely on our cumulative knowledge to survive. Once we lose all our technological know-how, chances are, most of us won't survive for long. So if you want to destroy humanity, start by destroying all their recorded knowledge, because humanity's power doesn't lie in their fists or their fangs. It lies in their ability to pass down cumulative knowledge.
**In today's world, you would also need to burn all their hard disk, data centers, and digital backups.