I completely agree with you about Nero (as I'd laid out on this and other threads), but the idea of a papal antichrist precedes the reformers by several centuries and is part of intra Catholic discourse. It assumes that the papacy is has central importance for the church. As I've said before, see chapters 4 and 5 of Bernard McGinn's Antichrist: Two Thousand Years of the Human Fascination With Evil.
Moreover, Luther's justification for being outside of the church had little to nothing to do with his belief in a papal antichrist. While the chronology is a little muddled, it seems that in his mature theology, when he believed in a papal antichrist, he had already come to the conclusion that he wasn't outside the church at all. Luther believed that the church is fully present wherever the word of law and gospel is rightly preached and the sacraments of baptism and communion are rightly administered. With that as the center of his doctrine of the church, the papacy (and the episcopate) became historical niceties, even preferred forms, but not essential. He had no theological need to justify being "outside" the church because as far as he was concerned, he wasn't.
Luther's belief in a papal antichrist had much more to do with his growing conviction (which sprung from his personal devastation at Pope Leo's bull Exsurge Domine) that the papacy had been corrupted by Satan into suppressing the doctrine of justification. Prior to 1520 Luther believed that the papacy was largely innocent of what he saw as a corrupt conspiracy on the part of certain cardinals and bishops to warp the doctrine of justification in order to raise funds through the illegal sale of indulgences; after 1520, he believed that the papacy was thoroughly implicated in this conspiracy, and was therefore a tool of Satan to take out the "doctrine of on which the church stands or falls," justification. Even after the publication of Exsurge Domine, though, he still was unsure whether the papacy was an unwitting tool of the devil or a willing participant, hence his dedication of his On Christian Liberty, published after his reception of the papal bull in late 1520, to Pope Leo X. I, personally, don't think he was fully committed to the view that the pope or papacy was antichrist until after his excommunication at Worms.
In any case, his doctrine of the antichrist didn't have much of anything to do with his doctrine of the church. I'm not saying I agree with his doctrine of the antichrist, or even that his thinking was entirely consistent. But his belief in a papal antichrist had much more to do with his understanding of the history of the doctrine of justification within the church than with his establishment of a counter-/para-church.