allowing all users to execute kill command on kubuntu 20.04LTS
Solution 1:
The only thing you are currently doing wrong is assuming that there is no need anymore to precede the command with sudo.
All users by default are already allowed to use the kill command. However, users only can kill processes they owned. To kill a process owned by root, users must precede the kill command by sudo to have it run with elevated permissions.
The change you performed to /etc/sudoers will allow them to execute sudo kill without having to enter a password. See Execute sudo without Password?