Hi and welcome to LQ!
You should always read the man pages for a command you are going to run and you're not familiar with first.For example man userdel
.When you run command userdel -r
some files owned by the user will still be on the system.Also it's better if you do not delete currently logged user.Try to use command
deluser --remove-all-files user
Replace user with the name of the user.If you read the man page for userdel
you'll see that in Debian you should use deluser
command.You can always reboot and see if the user still exists.