Does Catholic and Orthodox belief say 'antichrist' has already come and gone? It seems to be most Protestant denominations believe that right before the second coming, the antichrist will come, so he has not come yet. How is this different in Catholicism and Orthodoxy? Do they believe that he has already come and gone (e.g. perhaps he was Nero)?
Did Nero "sitteth in the temple of God, showing himself as though he were God" (1 Thessalonians 2:4)? Before Nero, was there "a revolt [αποστασια = apostasy] first," after which "the man of sin be revealed, the son of perdition" (ibid. 2:3)?

Cardinal Manning writes:


  St. Jerome, with some others, interprets this revolt to be the rebellion of the nations or provinces against the Roman Empire. … They have revolted, and no manifestation has appeared.


Thus, the revolt that must precede Antichrist is spiritual, not temporal.

Yes, Catholics believe Antichrist's reign is one sign of the end of the world. {The others are: a general preaching of the Christian religion, conversion of the the Jews, return of Enoch and Elijah, a great apostasy, perturbations of nature, universal conflagration, the Trumpet of Resurrection, and the sign of the Son of Man appearing in the heavens (source).}