Bouddica, as portrayed by Romans, already appeared in a more complex picture than we could expect it at first : like several classical depictions of the Barbarian warchiefs before and after her, she was depicted as a leader whom reasons for going to war went beyond simple savagery and innate rebelliousness.

Tacitus, being the main contemporary source (possibly thanks to an account given by Agricola, who was part of the events surrounding the revolts) depicted Boudica in broad brushes in Annals, besides a very short mention in Agricola, where he might have confused her with Cartimandua, the queen of Brigantes.

Annals

Prasutagus, king of the Iceni, famed for his long prosperity, had made the emperor his heir along with his two daughters, under the impression that this token of submission would put his kingdom and his house out of the reach of wrong. But the reverse was the result, so much so that his kingdom was plundered by centurions, his house by slaves, as if they were the spoils of war. First, his wife Boudicea was scourged, and his daughters outraged. All the chief men of the Iceni, as if Rome had received the whole country as a gift, were stript of their ancestral possessions, and the king's relatives were made slaves. Roused by these insults and the dread of worse, reduced as they now were into the condition of a province, they flew to arms and stirred to revolt the Trinobantes and others who, not yet cowed by slavery, had agreed in secret conspiracy to reclaim their freedom. It was against the veterans that their hatred was most intense. For these new settlers in the colony of Camulodunum drove people out of their houses, ejected them from their farms, called them captives and slaves, and the lawlessness of the veterans was encouraged by the soldiers, who lived a similar life and hoped for similar licence. A temple also erected to the Divine Claudius was ever before their eyes, a citadel, as it seemed, of perpetual tyranny. Men chosen as priests had to squander their whole fortunes under the pretence of a religious ceremonial. It appeared too no difficult matter to destroy the colony, undefended as it was by fortifications, a precaution neglected by our generals, while they thought more of what was agreeable than of what was expedient.

[...]

Like ruin fell on the town of Verulamium, for the barbarians, who delighted in plunder and were indifferent to all else, passed by the fortresses with military garrisons, and attacked whatever offered most wealth to the spoiler, and was unsafe for defence. About seventy thousand citizens and allies, it appeared, fell in the places which I have mentioned. For it was not on making prisoners and selling them, or on any of the barter of war, that the enemy was bent, but on slaughter, on the gibbet, the fire and the cross, like men soon about to pay the penalty, and meanwhile snatching at instant vengeance.

[...]

Boudicea, with her daughters before her in a chariot, went up to tribe after tribe, protesting that it was indeed usual for Britons to fight under the leadership of women. "But now," she said, "it is not as a woman descended from noble ancestry, but as one of the people that I am avenging lost freedom, my scourged body, the outraged chastity of my daughters. Roman lust has gone so far that not our very persons, nor even age or virginity, are left unpolluted. But heaven is on the side of a righteous vengeance; a legion which dared to fight has perished; the rest are hiding themselves in their camp, or are thinking anxiously of flight. They will not sustain even the din and the shout of so many thousands, much less our charge and our blows. If you weigh well the strength of the armies, and the causes of the war, you will see that in this battle you must conquer or die. This is a woman's resolve; as for men, they may live and be slaves."

Cassius Dio accounts were essentially similar to Tacitus own, even if the portrayal somewhat changes on the account of battles, for instance, but especially with a longest and first-person version of her speech, full of mention to Roman history and references to Nictoris and Semiramis (which beyond the rhetoric style makes it likely an invention from Dio) but some aspects should be underlined.

But, to speak the plain truth, it is we who have made ourselves responsible for all these evils, in that we allowed them to set foot on the island in the first place instead of expelling them at once as we did their famous Julius Caesar [...] As a consequence, although we inhabit so large an island, or rather a continent, one might say, that is encircled by the sea, and although we possess a veritable world of our own and are so separated by the ocean from all the rest of mankind  p89 that we have been believed to dwell on a different earth and under a different sky, and that some of the outside world, aye, even their wisest men, have not hitherto known for a certainty even by what name we are called, we have, notwithstanding all this, been despised and trampled underfoot by men who know nothing else than how to secure gain.

Needless to say, Iceni weren't mentioned by Caesar, a priori playing not worthwhile role against him; and Iceni were considered as allies and clients of Romans before the Rebellion; but Dio is depicting there a natio, an union of peoples otherwise divided, suddenly united genealogically and geographically; it is not necessarily the wrong viewpoint of an outsider, even if we don't really know how were regulated and conceived British people relations between themselves (considering Gauls or Celtiberians, a sense of community comparable, but not similar, to what Greeks felt about themselves, isn't to exclude).

However, even at this late day, though we have not done so before, let us, my countrymen and friends and kinsmen, — for I consider you all kinsmen, seeing that you inhabit a single island and are called by one common name, — let us, I say, do our duty while we still remember what freedom is, that we may leave to our children not only its appellation but also its reality. For, if we utterly forget the happy state in which we were born and bred, what, pray, will they do, reared in bondage?

But between the references that a Britton leader, would it be a queen, would probably not have said and expected to be understood at a war assembly, what really hammers the trope of the "noble Barbarian" holding a mirror to Romans, who failed to follow their mission civilisatrice, their mission to bring civilisation to the edges of the world because of their vices, is this part.

if, indeed, we ought to term those people men who bathe in warm water, eat artificial dainties, drink unmixed wine, anoint themselves with myrrh, sleep on soft couches with boys for bedfellows, — boys past their prime at that, — and are slaves to a lyre-player and a poor one too

Paying the price for their short-coming, Boudica identified herself as a goddess of Victory, Andraste/Andate, and she went on to

sack and plunder two Roman cities, and, as I have said, to wreak indescribable slaughter. Those who were taken captive by the Britons were subjected to every known form of outrage. The worst and most bestial atrocity committed by their captors was the following. They hung up naked the noblest and most distinguished women and then cut off their breasts and sewed them to their mouths, in order to make the victims appear to be eating them; afterwards they impaled the women on sharp skewers run lengthwise through the entire body. 3 All this they did to the accompaniment of sacrifices, banquets, and wanton behaviour, not only in all their other sacred places, but particularly in the grove of Andate. This was their name for Victory, and they regarded her with most exceptional reverence.

Both these authors would form the essential sources on which Boudica's representation would be based henceforth : a Barbarian Queen, wiser and more civilized than her savage and bloodthirsty army, strong of her right against vices that Romans fell to, but that good generals and governors overcame and eventually defeated as much as they defeated her (when the previously mentioned Cartimandua depicted as she indulged into her treachery against Brittons and in favour of Romans). Tacitus and Dio had diverging point of views about the queen, from a rather motherly and protective figure, to a warrior vamp, but these tended to mix up with time.