How do I delete a new Admin account and its data without knowing the old admin password? I used my friend's Mac by creating a new admin account.  Now I need to return it and want to delete the new Admin account I created without knowing his password.  In other words, can I delete my own Admin account?
First, create a new Admin user account as your account allows to make it. 
Login with the new admin account. 
Delete your original account and select the "Delete home folder" option. 


Easy way to delete your account with settings, not so easy to delete the new second account but your friend can do that with their admin account (hopefully)