"If you forgot the answers to your Apple ID security questions" click here --> https://support.apple.com/HT201485
To find out 'How to delete your Apple ID account', click here --> https://support.apple.com/HT208504
However, just as Apple will need for you to prove that you are the true owner of an account as they did with your security questions, so they will for you to do any action on your account including deleting it.
Security and your Apple ID - https://support.apple.com/HT201303 "Without proof of your identity ... and other carefully selected criteria(1), Apple Support can't help you perform any actions on your account."
Also worth noting is it may eventually disappear https://www.apple.com/legal/internet-services/icloud/en/terms.html
VII. B " In addition, Apple may terminate your Account upon 30 days’ prior notice via email to the address associated with your Account if (a) your Account has been inactive for one (1) year; or (b) there is a general discontinuance of the Service or any part thereof."