Even the Roman Empire is a kind of abstraction and the writer of Revelation zeroes in on a particular man, whom he is reluctant to name--perhaps because if he were to do so, he would be subject to the charge of treason and the punishment of execution. Cautiously he identifies the man in such a way that his more knowledgeable readers will know exactly whom he means and yet the law will not be able to touch him...
Commentators have considered virtually every possible candidate for the beast and the one most frequently mentioned is Nero.
(Asimov's Guide to the Bible, pg 551, Avenel Books, 1981)
Jgnat: Asimov also discussed 666 vs. 616, which he says can be attributed to an alternative spelling.