So it doesn't matter if I add user to sudoers file,I'll always have to preface command with sudo? :)

First sorry if i posted in wrong section
So, I want to add user to sudoers file to give him permission to del users (to use "userdel command".
I started bash and execute visudo. On the end of file I add new line so it looks like this
"user name" ALL = /usr/sbin/userdel
save the file and quit.
When I try to execute command as users specified in sudoers file I get error message "userdel: cannot lock /etc/passwd; try again later" and I must use "sudo" to del user.
Where I made mistake?
Thanks for help