Enemy nations
Large, powerful botnets will be a critical weapon in the next war fought between nation states. Without question, the United States, Russia, China, various European countries, and probably North Korea and others have extremely powerful worms ready to go. There’s little evidence of it, but it’s entirely plausible that these worms have already been deployed and quietly infect millions of computers, only to be activated the moment a war is declared. These are the nukes of the Internet age; on a moment’s notice, thousands or millions of computers could suddenly be tasked with crippling or destroying the information infrastructure of another nation.
This has already happened. The 2010 attack on Google and dozens of other companies in China was almost certainly military, judging from its sophistication. The governments of Estonia and Georgia were brought nearly to their knees by botnet attacks that may have originated at least partially with the Russian military. South Korea may have been attacked by a North Korean botnet in 2009. Your computer may already be a part of a botnet operated by the U.S. government and tasked with taking down the network of the next country to announce it has a lot of oil.