What does it mean when an author's purpose is literal?

It means that the author really means their purpose; it isn't a metaphor.

For example, if the author's purpose was to show that "In the End, Love Kills." (which, by the way, would make for a creepy book).

If it was figuritive, then Love wouldn't actually kill someone, but would put someone through enough misery that they are dead inside (emotionally, spiritually, etc.)

If it was literal, on the other hand...then yes, Love causes people's hearts and brains to stop working, or leaves people with daggers in their backs.