There are stories of Microsoft actually creating a few virii themselves, to make sure that people bought their products for "protection". There are also the stories of early McAfee anti-virus being extremely difficult to remove from any given Windows system, and rumors of the virii coming from their own networks.
It's all about the Benjamins. From those Nigerian scammers to the Chinese ransomware to those groups that hack data and lockdown systems demanding millions to release them to M$ and other companies releasing virii just to make more money, it all comes down to those who just want money. They use the power of technology to get more money. I don't think there's anything clean in the software world.