Did you know you can disable Windows Defender via Group Policy? Here’s how to disable Windows Defender via a GPO:
Enable the Turn off Windows Defender GPO located here: Computer Configuration -> Administrative Templates -> Windows Components -> Windows Defender.
There are a number of reasons you might want to disable Windows Defender, the most likely being that you have an alternative (3rd party) antivirus system that have installed and don’t want both running on the same machine.