Fridge / The Immortal (1990)

Fridge Brilliance:

• Why does the Dragon attack you at the end, even though you have the amulet? Why does it pause its attacks the moment you hold up the amulet? Why doesn't it attack Mordamir, when the latter appears? And why on Earth doesn't it attack you after Mordamir takes the amulet from you?
  It's simple: the amulet doesn't have any magical power other than destroying its own bearer and the Dragon. It doesn't magically prevent the Dragon from attacking you. So, as long as you have the amulet in your inventory, where the Dragon can't see it, it attacks you, because you are a human invading its lair, and bitter experience has taught it every human is its enemy. Then the Dragon gets exhausted from attacking you, giving you an opportunity to take the amulet out and hold it up. The Dragon can now see you have the weapon to destroy it, so it knows it's at your mercy. But you don't use the amulet, and so the battle reaches a stalemate, which prompts Mordamir to appear and angrily shout at you to read the runes. The Dragon is still visibly exhausted at this point, and that's why it doesn't attack Mordamir right away. Then Mordamir quickly takes the amulet from you before the Dragon can recover. Seeing all that, the Dragon, who is an intelligent creature, understands you are not its enemy, and that's why it doesn't attack you anymore and just watches as Mordamir fails to kill you. Then you take the amulet back, and the Dragon sees its opportunity to take out its ancient enemy. After the battle is resolved, it goes away and leaves you alone, probably even grateful to you for bringing Mordamir to him, ending their millenium-long feud.
  OK, maybe it wasn't exactly simple, but it's plain and logical.