Killed my phone by USB tethering I couldn't connect to a WiFi network over my laptop so I used my phone to connect to the WiFi network instead and activated USB Tethering. When I started, the battery was empty. I started charging it via the USB cable connected to my laptop and I enabled USB Tethering.

The problem arised when the phone said that more power is being used than charged and that I should plug the phone into the wall socket. I did so and the battery wouldn't charge past 4%. It charges so slowly. When I'd turn on the phone or unplug it, the battery level would drop to 1%. I think I've destroyed it.

Has this ever happened to anyone? Is this a software glitch or a hardware problem? Have I destroyed both the phone and the battery?
USB tethering by all means should not cause this and I do not believe that your battery is dead. Instead, it's more probable that you stumbled upon a glitch within the OS.

Back up all of the data in your phone including photos and other things in your external storage, then do a factory reset on the phone. If this does not resolve the issue, there may be more serious underlying hardware or software issues. Contact the manufacturer if a factory reset does not fix the issue.