We really did not expect anything from Security.nl: you can easily delete your account. Please be logged in, click the delete link, fill in your password at the bottom of the page: tick 'Unlock', then continue (button at the bottom) and you're ready.
URLS:
https://www.security.nl/
https://www.security.nl/profiel/wijzig
Please note that AccountKiller.com is not associated with the business listed on this page. AccountKiller provides easy-to-understand instructions and guides on how to cancel charges and relationships from the list of businesses on our site, we have no affiliation with these businesses.