Book Reports / Lord of the Flies and Macbeth - essay on a comparison between 2 novels [4]

Exams are coming up,
and i'm gonna have to write an essay about the following:
compare the two characters that support the protagonist and explain how their support leads to their ultimate downfall.
the two books compared are: Lord of the Flies (William Golding) and, Macbeth (Shakespeare, duh).

the two characters being compared: Piggy (supports Ralph) and Lady Macbeth (supports Macbeth).

ok, so i need to brain storm, i need some solid ideas.
we're gonna be able to bring a page full of notes, so i can fit my thesis, closing paragraph, and some quotes. i need 5 paragraphs.

if anyone has read these 2 books, the please tell me how i can compare them, and maybe some examples, i really want to do good on this exam.