How do I disable Windows Defender? Every time I turn it off, it comes back on by itself.

You can disable Windows Defender either by using Group Policy or the Windows Registry.

Group Policy
Computer Configuration > Administrative Templates > Windows Components > Windows Defender
On the right: Turn off Windows Defender
Select Enabled

Windows Registry
HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows Defender
New, DWORD (32-bit) Value
Name the key DisableAntiSpyware
Set the value from 0 to 1