Aeneas's home city is destroyed in flames, and he has no future there. The only way he can do good for himself, his descendants, and his people, is by hitting the road.
Throughout all of these books, Aeneas has to overcome various challenges that threaten to derail him on his quest. This leads up to the ultimate challenge of going down to the underworld, which only ends up making him more gung-ho about finishing what he's started.
Just when Aeneas thinks he's home free, he learns he'll have to meet one final challenge – war against the Italians – before he can claim the kingdom promised to him by the gods.
The challenge of warfare becomes a reality in these books. Aeneas has to endure the pain of seeing Pallas, whom he had taken into his care, die at the hands of Turnus.
By killing Turnus, Aeneas has surmounted the final challenge. That said, it's a pretty abrupt and brutal end to the adventures we've been following up to this point. We're expecting to see a scene of reconciliation – say, Aeneas and Lavinia getting hitched – but Virgil doesn't take us there. Thus the poem ends on a note of uncertainty.