I'm just browsing around the old archives and peforming necromancy on a few topics here and there . As for this poll, I decline to vote because I don't think any of these emperors were truly "evil". As has been pointed out, defining the term "evil" is problematic to begin with, but even in the popular sense of the word, I don't see anyone among this list who would qualify as "malicious" or "ill-intended".
To start with Caligula, while he may have done some truly outrageous or cruel things during his brief reign, he seems to me to have been genuinly insane. And if he was, I mean mentally ill, he was obviously not in control of his actions and unfit to rule right from the start.
Nero doesn't strike me as particularly evil either, but his reign has suffered much from the slander of posterity. Popular history would like us to believe that Nero fiddled while Rome burned (or in fact, set it on fire himself) but for one, the fiddle wasn't even invented yet , and according to Tacitus, he was in Antium at the time the fire broke out. Of course the construction of the Golden House may have been a bad move, and probably did little to dispell popular rumors, as did the fact that, when the fire had nearly subsided, it flared up again in Tigellinus' backyard.
As for the murder of his mother, well, there seems to be no doubt he gave the order, and it is hard to find a more vicious act among the other emperors listed. Then again, I doubt there was much affection between mother and son; their alliance and later rivalry may have been purely political, and Agrippina seems to have been equally ready to replace Nero with Britannicus.
Aside from matricides, the first half of Nero's reign was fairly peaceful, and his administration competent. Afterwards he got progressively lazy and decadent. Notice how you can actually guess the year of mint on Nero's coins from the number of double chins in his portrait. Even his statues got fatter. But I digress. A lot of his companions, like Tigellinus or Nymphidius Sabinus strike me as far more evil than Nero himself. And in his late reign he was mostly preoccupied with his artistic pretensions.
Domitian was said to be cruel and paranoid, but modern historians agree that generally, he ruled well. From a certain point of view however, he may actually have been the most evil of the five listed here. Because whereas the others were either mad, completely uninterested in the office, or otherwise dim-witted, Domitian was calculated and intelligent. There are no unhappy childhoods, brain fevers or malignant freedmen to point to. Domitian was in complete control. Still, did it make him evil?
As far as Commodus is concerned, it was Cassius Dio (an eye-witness to his rule) who described him as "not naturally wicked but, on the contrary, as guileless as any man that ever lived. His great simplicity, however, together with his cowardice, made him the slave of his companions." So he seems to me to have been simply a man of low intelligence, genuinly believing that he was a demi-god and uninterested in the state administration.
Heliogabalus finally is a different story. As with Nero, it may be impossible to separate fact from fiction on his reign. Cassius Dio's text is mostly based on hearsay, and the Historia Augusta 75% fiction. Herodian seems to be the most reliable, and he mentions little of the lurid extravagances recorded by the former two, in short describing him as "an empty-headed young idiot". Heliogabalus seems to have been mostly interested in his religious duties: dancing the night away and worshipping his piece of meteorite rock. Maybe he did indulge in the decadence recorded by the Historia Augusta, but I can only say: that's what happens when 14 year old boys are suddenly led to believe they are Gods. The entire episode is really a complete mockery of the Roman emperorship.
The thread that seems to emerge from the five emperors mentioned here is simply that they all attained their power at too early an age. Caligula was 25, Nero 17, Commodus 19 and Heliogabalus, shockingly, 14 years old at the time they became emperor. It's no surprise that the most capable man here, Domitian, was also the oldest to acceed to the throne (30y).
Curio Agelastus wrote:and although he wasn't an emperor, Sejanus was fairly villainous.
I beg to differ I think in the end, Sejanus was no more villainous than Tiberius was. Both took advantage of each other to a certain extent: Tiberius used Sejanus to wipe out his own political opponents (Agrippina's faction) and Sejanus used Tiberius to amass power. Neither one of them could be faulted for bad administration, and although Tacitus devotes a substantial part of his Annales to the notorious treason trials, it's said that the true number of these "political murders" was blown out proportion by him.
There is little evidence that Sejanus actually did conspire against Tiberius, with only vague notions of a "plot" in the ancient sources, and the facts pointing toward Tiberius himself as the more likely instigator of his prefect's downfall. Of course, I'm not ready to let him off the hook as far as the murder of Tiberius' son Drusus is concerned, but if you're looking for real evil here I'd say look no further than Livilla.
And while I'm thinking about it, several women from ancient Roman history strike me as far more evil than any of the men ever were: Livia, Livilla, Agrippina the Younger,... Of the emperors it can be said that they were tyrannical, and of their subordinates - prefects like Sejanus, Tigellinus or Laetus - that they were merely unscrupulous or opportunistic. But the women,... sometimes there's no guessing as to what their motives were