You know how sometimes in cartoons a little devil and a little angel will appear on a character's shoulder and give him contrary opinions about what to do? Well, if William Blake were trying to write some poems and if Mother Goose appeared dressed both as an angel and as a devil, the angel Mother Goose would tell him to write a pretty, neat little poem that has a nice, happy story in it with plenty of good rhymes. The evil Mother Goose, on the other hand, would dictate "The Sick Rose." Her poem would be about a worm destroying a rose and wouldn't rhyme as well as the angelic Mother Goose's poem would.
Take the first stanza as an example:
O rose thou art sick, The invisible worm That flies in the night In the howling storm.
The only two words that look like they should rhyme are "worm" and "storm," but they don't really rhyme, they only look like they do. That devil Mother Goose sure is sneaky. The second stanza is similar:
Has found out thy Bed of crimson joy. And his dark secret love Does thy life destroy.
Now at least in this stanza our evil Mother Goose has learned how to rhyme ("joy" and "destroy") but true to form she makes it a naughty rhyme; "joy" and "destroy" rhyme because that's precisely what the bad Mother Goose wants to do: destroy the joy of nursery rhymes, or at the least the joys that her counterpart, the good Mother Goose, always writes about. And she does a pretty good job of it; her poem manages to find a way to talk about sex, violence, and death.