Anti-virus (AV) software is basically a root-kit that you trust to run on your computer to protect it from the "bad guys". By installing AV software you are allowing remote administrative access to your PC. Every day AV reads every file on your computer and can send a report back to the AV manufacturer of what files are on the computer or send the files themselves. You really don't know what's in the AV software package and the AV manufacturer can do whatever they want to your PC.

McAfee AV was popular for years, he could have targeted specific people or run a wide dragnet to see what people had on their PC's. Don't trust any software vendor.