There can be little argument that Chief Justice Taney's decision in the Sanford Case was an odious one, and one might also argue that Taney, as a slave holder, was predisposed to rule against Scott. Be that as it may, one must consider the time and circumstances of the case, at a time when slavery was considered a legal and constitutional institution, horrible though it might be. That being the case, Taney's decision was within the bounds of the applicable law, and therefore correct at the time.
It should be mentioned that Taney did not treat the matter lightly. He wrote in his opinion:
This is certainly a very serious question, and one that now for the first time has been brought for decision before this court. But it is brought here by those who have a right to bring it, and it is our duty to meet it and decide it.
Having said that Taney went on to rule that Scott was not a citizen within the meaning of the phrase as used in the Constitution or the Declaration of Independence:
In the opinion of the court, the legislation and histories of the times, and the language used in the Declaration of Independence, show, that neither the class of persons who had been imported as slaves, nor their descendants, whether they had become free or not, were then acknowledged as a part of the people, nor intended to be included in the general words used in that memorable instrument.
It is difficult at this day to realize the state of public opinion in relation to that unfortunate race, which prevailed in the civilized and enlightened portions of the world at the time of the Declaration of Independence, and when the Constitution of the United States was framed and adopted. But the public history of every European nation displays it in a manner too plain to be mistaken.
Taney's opinion appears supported by his interpretation of the Constitution.
He futher ruled that the Missouri Compromise, although abrogated earlier by Congress, was itself unconstitutional. Since slaves were property, their owners had certain rights under the Fifth Amendment to the Constitution which no state could violate. Accordingly, no state could outlaw slavery such that a slave owner could not bring his personal property, that is his slave, into the state.
Through the eyes of hindsight, the ruling is terrible; however at the time it met the spirit and letter of the Constitution.