What's the purpose of the first paragraph?

I think that the purpose of the first paragraph is to convey to the reader that the narrator is unreliable. That the narrator is unreliable is clear from the start when he protests a bit too robustly that he is not mad. That he is to be reckoned with comes with his admission that tomorrow is the scheduled date of his death. This unreliability is only further enhanced with his curiously unlikely admonition that the string of events bringing him to his current state were the result of nothing more sinister or inexplicable than mere cause and effect.