Actually it is a good example of an”Activist Decision” on the part of the Supreme Court. The only issue before the Court in Scott V. Sanford was whether Scott could sue in Federal Court for his freedom. This could have been answered in a one page opinion, i.e., Dred Scott was not a citizen of the United States and therefore had no standing to sue in Federal Court for his freedom. Instead we got copious pages of opinion including that The Missouri Compromise was unconstitutional, that was not an issue presented in the suit to the court. The Federal Government has no Constitutional authority to interfere with slavery in any state where slavery is legal. That was also an issue that was not in the suit. The Federal Government must enforce the Fugitive Slave Act. Again that was not an issue in the suit. And finally, there is no way that a slave can become a citizen of the United States. That per se was not in the suit. Only Dred Scott’s status as a slave and citizenship was the question to be interpreted per the Constitution. That seem to be pretty activist jurisprudence to me. Okay, you make a good case that in terms of legal process, Tanney went far beyond what was necessary. Yes, he could have dismissed it on the basis that Scott was not a citizen, however, the term "Judicial Activist" is normally used to describe a condition in which a Judge interprets a law in a manner never intended by it's creators, such as "Pornography" being part of "Free Speech" of "Abortion" being derived from the 14th amendment.
Taney may have gone beyond what was the necessary legal process, but I don't see where he invented any new law. He may have been an "activist" in so far as he used this case as a vehicle to clarify the existing law on larger points than just the status of Dred Scott.
Yes, he would appear to be an activist in that regard, but this is very different from what we normally consider "Judicial Activism."