Taney went way further than the matter in front of the court at the time. His meanderings, reasonable and unreasonable, were not germane to the question of Dred Scott's ability to sue in court and were made in dicta.
From what I have read, people say he was intent on making a political statement through the power of the court, and that's why he threw everything into it but the kitchen sink.
It just happened to be the case on which he hung all the other stuff that he didn't like over the years.