Why is the username still displayed even when it's been deleted using 'userdel' [duplicate]

I added a user with the adduser command and then deleted it withuserdel, but the username is still displayed in the home directory and in the switch account section.

1. Why does this happen?
2. How do I get rid of it?

By default, userdel only deletes the user from /etc/passwd. To remove their home directory too, you pass the -r or --remove flag.

Once the user has been removed using userdel, you cannot retrospectively run userdel -r user to remove the home directory; instead you need to delete the user's home directory yourself, for example by running

sudo rm -r /home/user

replacing user with the username you want to remove.

If there is an entry for the user in /var/mail, or whatever directory is defined for MAIL_DIR in /etc/login.defs (run grep MAIL_DIR /etc/login.defs to check), you can delete that too.

Deleting the home directory should fix the switch account menu too, but I couldn't reproduce your issue on my Ubuntu MATE system. Perhaps rebooting after removing the user's home directory is also needed for you.

See also the more flexible tool deluser.