Final answer:
The sight of Pallas's sword-belt on Turnus incites Aeneas's rage, leading to the killing of Turnus as it symbolizes the need for vengeance for Pallas's death.
Step-by-step explanation:
The "fateful sword-belt" that stirs Aeneas's rage and prompts him to kill Turnus is that of Pallas, a young ally whom Turnus had slain earlier in the epic.
When Aeneas sees the belt on Turnus, it is a stark reminder of his duty to avenge the death of Pallas, which drives him to act with fury.
The belt, a spoil taken by Turnus after killing Pallas, symbolizes the personal loss and the breach of the codes of warfare and honor, fueling Aeneas's wrath and solidifying his decision to slay Turnus despite earlier contemplations of mercy.