i used shreddit too, took a few minutes but right away it started to delete my newest posts, after about 10 minutes it deleted everything, then i went in and deleted my account. easy.
On my phone so I can't dig out links, but there are two tools being recommended: Redact and PowerDeleteSuite. Both allow you to edit the comment/post before deleting, because if you straight-up delete the message Reddit reserves the right to the message content as it stood at deletion.
Good point about deleting or edit your comments before deleting the post, I initially started by going through mine manually before using Redact and a few slipped through the net.