Other examples of proprietary malware
Nonfree (proprietary) software is very often malware (designed to mistreat the user). Nonfree software is controlled by its developers, which puts them in a position of power over the users; that is the basic injustice. The developers often exercise that power to the detriment of the users they ought to serve.
This document reports instances where proprietary software is dishonest or conceals deception or trickery.
Many proprietary programs secretly install other proprietary programs that the users don't want.
“Dark patterns” are user interfaces designed to mislead users, or make option settings hard to find.
This allows a company such as Apple to say, “We allow users to turn this off” while ensuring that few will understand how to actually turn it off.
A top-ranking proprietary Instagram client promising to tell users who's been watching their pictures was in reality stealing their credentials, advertising itself on their feed, and posting images without their consent.
Volkswagen programmed its car engine computers to detect the Environmental Protection Agency's emission tests, and run dirty the rest of the time.
https://www.petri.com/volkswagen-used-software-to-cheat-on-emissions.
In real driving, the cars exceeded emissions standards by a factor of up to 35.
Using free software would not have stopped Volkswagen from programming it this way, but would have made it harder to conceal.