Oh lord, Sallust's opinion? I've been working on Sallust for years and I can't work out his opinion about much of anything. This is a little off topic, but I have worked a little on Sallust's view of history in the Catiline. First he says writing history is hard because people who read history don't bloody listen, then he says don't listen to historians because they are liars. So both the writers of history and the readers of history are trouble, and he has to write a history to let us know this?

Back to the Jugurtha. There are a couple reasons to make Jugurtha a complex character. Whether Sallust intended any of these is something I don't suppose we can really know without asking him, but for what it's worth....

First you have to remember Sallust's goals for this piece:

because then for the first time resistance was offered to the insolence of the nobles — 2 the beginning of a struggle which threw everything, human and divine, into confusion, and rose to such a pitch of frenzy that civil discord ended in war and the devastation of Italy.

It's the nobles who are the bad guys, not Jugurtha. He's what Thucydides might call a "proximate" cause; he exposes much greater problems in Rome than some petty succession dispute.

Compare Jugurtha's education to Cyrus the Great's. This is what Xenophon has to say about Cyrus.

And besides, they teach them self-restraint in eating and drinking; and it greatly conduces to this also that they see that their elders do not leave their post to satisfy their hunger until the officers dismiss them; and the same end is promoted by the fact that the boys do not eat with their mothers but with their teachers, from the time the officers so direct. Furthermore, they bring from home bread for their food, cress for a relish, and for drinking, if any one is thirsty, a cup to draw water from the river. Besides this, they learn to shoot and to throw the spear.

And Jugurtha:

As soon as Jugurtha grew up, endowed as he was with physical strength, a handsome person, but above all with a vigorous intellect, he did not allow himself to be spoiled by luxury or idleness, but following the custom of that nation, he rode, he hurled the javelin, he contended with his fellows in foot-races; and although he surpassed them all in renown, he nevertheless won the love of all. Besides this, he devoted much time to the chase, he was the first or among the first to strike down the lion and other wild beasts, he distinguished himself greatly, but spoke little of his own exploits.

Sallust is setting Jugurtha up on the model of Cyrus, whom practically everyone in antiquity loved. (Something in the back of my mind is telling me Scipio Aemilianus also spent more time hunting in this youth, but I can't find that. If anyone has that passage on hand I appreciate the reminder). EDIT: Ah, Polybius book 31. It's a long passage. Further up in this passage Polybius makes a point that Scipio led a sober life. Anyway, on hunting:

When the war had been brought to a conclusion, Aemilius, thinking that hunting was the best training and amusement for the young men, placed the royal huntsmen at Scipio's disposal, and gave him complete control over the preserves. 6 Scipio, availing himself of this and regarding himself as being nearly in the position of king, spent the whole time that the army remained in Macedonia after the battle of Pydna in this pursuit, 7 and, as he became a very enthusiastic sportsman, being of the right age and physique for such an exercise, like a well-bred dog, this taste of his for hunting became permanent. 8 So that when he arrived in Rome and when he found in Polybius one equally devoted to the chase, all the time that other young men gave up to law affairs and greetings, spending the whole day in the forum and thus trying to court the favour of the populace, 9 Scipio was occupied by the chase....

So we've got another parallel to hunting as a morally superior alternative to practicing law for the education of young men.

It's also worth noting Sallust describes the Numidians as descendants of Persians.

Ok, on to the bribery.

began to be afraid of the Roman people and to despair of escaping their anger except through the avarice of the Roman nobles and his own wealth.

Again, it's the nobles that are the problem here; yes Jugurtha treacherously seized Numidia, but Cyrus had done that in Persia too. Same model. Here, Jugurtha knows the real weakness of Rome is being able to exploit the nobles. And it works!

The first really nasty things said about Jugurtha are in the speech of Adherbal, so it's not exactly omniscient narrative here.

Sallust gets nasty around the sack of Cirta. Something went wrong there. It makes no sense for Jugurtha to kill the Italian population; that would almost certainly drive Rome to war, the thing Jugurtha's been trying to avoid all this time. The Italians successfully negotiated Adherbal's surrender and ended a tricky siege Jugurtha didn't have time for. I'm not sure what Sallust was getting at, except there seem to be echos of Saguntum here.

Finally, the heroic villain type is one Sallust likes. Here's his description of Catiline:

His body could endure hunger, cold and want of sleep to an incredible degree; his mind was reckless, cunning, treacherous, capable of any form of pretence or concealment.

This guy's another Roman noble. He's got all these good qualities that become corrupted by primarily the desire for money. He makes this explicit:

For avarice destroyed honour, integrity, and all other noble qualities; taught in their place insolence, cruelty, to neglect the gods, to set a price on everything.

This is the problem. You can take a good guy, a guy who has nearly the same background as Cyrus, who can be corrupted by the corrupt Roman nobility. The rest of it is Marius, the political outsider with no famous ancestors, who is the one with enough skill and integrity to win (well, maybe, depending on whether you ask Sulla) the war.

And it wouldn't make a good story if Marius had to go fight Jugurtha if he were a chump, right?