Yeah many free anti-virus programs are actually a sort of malware/spyware themselves. They have access to about every file on your computer and since nowadays these anti-virus programs works with the cloud, they regurely send your files to their cloud server to scan them. Many free anti-virus programs datamine on your pc, annoy you with pop-ups, give lots of false warnings (some even throw away your files without mentioning it to you), install other programs if you carelessly click through the next buttons during install etc. etc.

That reminds me of McAfee which reminds me of this classic: https://www.youtube.com/watch?v=bKgf5PaBzyg

ps: sry mods for going offtopic here but I think this is an important subject