This is a better explantion from someone on IMDB:
1. Balkin committed all of the murders (Except the first murder. Corso's partner was killed by the "albino"; you could see his feet in the window right before the witch chased him off.) to get the etchings from all three books; this is obvious because at the end of the film Balkin has all nine etchings by "LCF" in his possession from all three books.
2. Johnny Depp was Lucifer LOST in the human realm; you can see this for several reasons. One, the nature of the keys, the purpose of these etchings created by LCF at an earlier time are to act as a sort of 'popcorn' trail from the earthy realm back to his own realm and are the keys to the ninth gate, or the realm of Lucifer. All of the events represented in each key (etching) was happening in real life to Corso (Depp). The woman helping him was the witch...early on in film in the lecture scene Balkin makes reference to "helpers" of Lucifer should he (Lucifer) become entangled in the human realm (and need help returning to his realm).
These helpers were witches that Lucifer would endow with certain gifts (power and youth; remember the helping-woman's physical strength, and how she was growing younger at times during the sex scene at the end.).
Corso also had the marks of the beast (fleshy weaknesses) in the scriptural sense, these were his addictions, alcohol and cigarettes, fornicating (sex outside of wedlock) and his unscrupulous business dealings, and he did not have any close personal relationships (aside from his business partner who was in his life to locate buyers to help him (Corso) make money) but lived for money.
Visually and characther-wise there were some additional clues to Corso's true identity as well, he was unkempt, wore a goat-tee beard and also seemed indifferent and aloof. Another clue was the fact that Corso was able to make the witch bleed, she took the blood and made three bars on his forehead, in the modern myth, this marking of three or of three sixes on the forehead is the mark of the beast.
Now, if you pay really close attention to what Balkin is saying in his lecture in the first part of the film then you will be able to figure all of this out. At the end of the film Corso is returning to his realm, opened only after he has experienced the ninth key. The woman in the ninth-key-etching is in fact the same woman helping him.
And in the ninth etching/key she is riding a three-headed beast (the symbol for Lucifer after his fall from grace) in the nude, this is a representation of the sex scene in which Corso is in the place of the three-headed beast therefore Corso is Lucifer. However, even in the etching, the beast is locked out of the castle, his realm, the ninth gate, and it is the witch who holds the book in one had and points the way towards the gate with her other hand. At then end of the film after the witch has recieved more power (youth via sex with Lucifer/Corso, also another popular imagery whereby witches gain power from Lucifer) she then points the way for Corso/Lucifer to find entry into his realm by leaving a copy of the fake etching on the car window with the name of the location of the real etching written across it. This chain of events fufills the requirments of the covenent between Lucifer and his witch-helper depicted in the last key.
This film is an allegory of the story of Lucifer, who, after his fall from grace, becomes so embroiled in fleshy, earthly, i.e."beastly" pleasures that he forgets where he comes from or how to return home. Satan and the devil are not and never were Lucifer, and vice versa. You will also notice that Satan and the devil are not mentioned in the film. Lucifer is a modern embellishment created circa 539 a.d. by Emperor Justinian I, The Justinian Code, in Constantinople...whereby the systematic interpolation of certain scriptures into new teachings (burning in hell, etc.) designed to scare/intimidate folks into paying their taxes (tithing) was undertaken.
This film was not meant to be obvious, but rather esoteric and subtly symbolic, so if you keep this in mind and pay close attention the film will make sense.
LB.