This *should* be easy. Follow the direct link supplied here. Then click on the ‘Cancel Account’ button to delete it.
However, at the time of testing, we couldn’t complete the process. If you are experiencing similar problems, leave a comment. For now SL is put on the grey list.
URLS:
http://www.secondlife.com
https://secondlife.com/my/account/cancel.php?lang=en-US
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.