Thucydides holds up two scenes for us: a speech by Pericles and a record of discussions between a delegation of Athenians and a delegation from the island of Melos.
The speech by Pericles is complex: on the one hand, he mainly makes some attempt to paint the Athenians with an element of the moral high ground, but on the other hand, he occasionally also lets some of their merciless aggression show. One certainly gains the impression that he is doing his best to present Athens as noble, and he uses the words "virtue" and "virtues" a number of times. The matter is compounded by the fact that he is giving this speech at a funeral during the Peloponnesian War, which the Athenians can hardly paint with the brush of ethical righteousness: if Athens wins this war, it means that they can continue the financial exploitation of the other Hellenic communities.
The record of the negotiations with the island of Melos is more direct in showing the Athenian motives: the Athenian diplomats - one is tempted to call them thugs - begin by saying that they're not even interested in discussion justice, and acknowledging that their aggression against Melos is largely unprovoked. The Athenians will make no effort to justify their actions, they simply point to their military superiority and make demands.
Thucydides presents us with both events; he wants us to wrestle with the contrasts between the high-sounding attempts by Pericles to paint Athens with a virtuous brush, and the blunt reality of the military annexation of Melos.