To expand on what others have said, be aware that the game of Magic has its own glossary of words. 'To cast from your hand' has a very specific definition, involving many steps, like picking the a card in your hand, placing it on the Stack and paying its cost, to name but a few. This is much more complex than the simple displacement of cards.
Beacon of Unrest is a spell. When it resolves, all it asks you to do is to displace two cards:
-displace one card from the Graveyard to the Battlefield (Dread Cacodemon)
-displace one card from the Stack to the Library (Beacon of Unrest itself)
And that's it! The only card you actually cast is Beacon of Unrest; the Demon card simply gets displaced, therefore its ability never fires off.
Thanks, it makes more sense now.