If you are unable to stop the Windows Defender service on Windows 10, don’t worry – this is normal. To stop the Windows Defender service, you have to disable Windows Defender in group policy. This is a security feature of Windows Defender that ensures viruses can’t just stop the service.
To stop the Windows Defender service, click Start and type “group policy”. Click Edit Group Policy and navigate to Computer Configuration -> Administrative Templates -> Windows Components -> Windows Defender. Double click on Turn Off Windows Defender then select Enable and click OK. Restart your PC or run gpupdate /force in a command prompt to turn off the Windows Defender service.
Note that some testing I’ve done with Antivirus software on Windows 10 shows that many antivirus programs (OK, I’ve only tested 2 so far!) are unable to respond to a virus in real-time as fast as Windows Defender. In my tests with an eicar test file, Windows Defender detected the file as soon as I tried to run it and removed it. Avira (my usual goto free antivirus software) didn’t remove the file and took some time to detect that I was running the file. Malwarebytes didn’t detect it at all (Avira got in there first). In my test I had all 3 antivirus products installed simultaneously, prior to disabling the Windows Defender service.