He never wanted to be Emperor. He thought himself unworthy. If Auerlias had made his will and set up his succession publicly, Livius would have accepted it. But the Emperor's death and the lack of any physical evidence to his plans meant he would have to take power illegaly, and he was too noble to do that.
His loyalty to Rome made him refuse to join the rebels because it would put Rome into civil war and probably destroy the country. At the time, invading Rome was something of a no-no for any Roman soldier. As Crassus said in SPARTACUS over an exception: "Sulla! To the infamy of his name!"
Remember what the turncoat general said about Aurelias being a good man but having no understanding about the soldier and their own financial interests about the rewards of war (looting the enemy)? If we view Livius as Aurelias' true successor, then his nobility would have refused to have the capital looted, much to his army's chagrin. Besides, it was a matter of doing it first. Commodus gave the army the gold first.
Again he never dreamed about becoming Emperor. And he didn't allow Rome to fall into 'beep'. At that point, Rome was already in the middle of 'beep' creek with the paddle broken. The point of the ending is that Rome is lost and at the beginning of fall (hence the title). All the noble people are dead and killed, the remaining people are the corrupt ambitious calling dibs on the throne & the sycophants (who had killed one Caesar and would do it again). Even though Livius' turncoat general made him Emperor he was only doing it for some self-interested reward. How long will his renewed loyalty last before some ambitious sleazebag gives him a better reward that Livius won't give him? How long will Livius sit on the throne before someone gives him a poisoned apple? (Heck, if you noticed, a victorious Commodus would not have come out of that arena alive) And judging by Livius' first action of crucifixion (an act Commodus would do, an act Livius refused to give to the rebels), it's clear he's no longer morally fit to steer things to right. Livius and Lucilla understand this too well. It is too late: Rome IS Lost. It is too corrupt for them to save, and any attempt will leave them corrupt or murdered themselves. They are not leaving a wounded animal to death. The animal is already dead.
At the time, invading Rome was something of a no-no for any Roman soldier. As Crassus said in SPARTACUS over an exception: "Sulla! To the infamy of his name!"
Not really. Invading Rome was a no-no during the republic, but Sulla broke that taboo. You mention Sulla, but he was only the first to invade Rome. By the time of Aurelius and Commodus, the city of Rome had been invaded by armies under the command of Sulla, Gaius Marius, Sulla a second time, Julius Caesar, Augustus and Vespasian. I might even be forgetting one or two (Vespasian invaded Rome during the year of the four emperors, but I don't think any of the other three actually invaded the city). After all this, invading Rome was par for the course for people who felt they had the chance to seize power.