I very much doubt the idea Nietzsche went insane due to syphilis. Syphilis progresses slowly. This was a sudden mental breakdown. What follows is my idea of what happened.
Nietzsche, like all of us, began life with a highly selfish perspective, but whereas others usually give it up somewhere along the way, as they recognize the primacy of the community over themselves, Nietzsche invented a highly creative way of holding on to childish evaluations. (As did his admirer, Ayn Rand, later.) He separated himself from all others and erected an entire philosophy to wall out his growing sense that the community mattered more than he did. His attitude was "If there were gods, how could I bear not to be a god. Therefore, there are no Gods." The attempt reaches a climax in his final complete book "The Antichrist", where he abandons all restraint in his criticism of a religion that identifies selfishness with evil:
Nietzsche wrote:This eternal indictment of Christianity I will write on all walls, wherever there are walls -- I have letters to make even the blind see. I call Christianity the one great curse, the one great innermost corruption, the one great instinct of revenge, for which no means is poisonous, stealthy, subterranean, small enough -- I call it the one immortal blemish of mankind.
After completing the Antichrist, Nietzsche worked on his autobiographical book "Ecce Homo". Here he reveals that he has been almost constantly in pain and that his thinking is
clearest when he is in pain. This is the only indication he gives that a breakdown may be looming. The pain is due (I think) to an internal conflict between the part of his mind that he has organized to express his philosophy and a more mature part that has been incubating with him for years, trying to wrest control. As he reviews his life, the struggle to hold on against mature evaluation gets increasingly difficult for him. When he sees a man beating a horse, a simple illustration of where the personal will to power leads, that dam breaks. His defenses fall, as he rushes to protect the horse. The mature mind takes over, but it is too late. Nietzsche loses his ability to think and speak coherently. He is declared "insane", because he finally went sane. He never fully recovers. Nietzsche was where Peter Pan's refusal to grow up leads - Never-never land.