How to delete a locked app using superuser privileges?
Read the shell script:
/Library/Application Support/org.pqrs/Karabiner-Elements/uninstall_core.sh

or

https://github.com/pqrs-org/Karabiner-Elements/blob/master/src/scripts/uninstall_core.sh

In summary:

Unload the kernel extensions.
Clear immutable flags nouchg,noschg from the apps.
Remove the files from /Library and /Applications. Remember we used sudo with the script.
Refresh kernel extension cache: kextcache.
https://github.com/pqrs-org/Karabiner-Elements/blob/master/src/scripts/uninstall.sh
Kill all orphan/ background processes if any.
Prompt you to restart the computer.