Some antivirus software can be fairly annoying to uninstall. Rather than going into the list of programs within your PC or Mac and uninstalling from there, the antivirus software can be embedded into your system that it technically remains on your computer even after going through the uninstallation process.
A lot of these companies have their own specialty removal tool that will allow you to fully uninstall their program from your computer. So if the next time you try to uninstall some antivirus software and it seems to remain on your computer, do a quick Google search for the software's removal tool. You can find the software on the company's support page usually.