Taney became a very arrogant man who was among the first activist Justices to 'legislate' from the bench'. His Dred Scott decision blatantly ignored both the letter and spirit of the Constitution as well as completely ignoring established precedence to the point of rewriting history to fit his political preference.
Dred Scott was such an atrocious decision. As you say, it was wrong both historically and on legal precedent.
For one thing, he flat out claimed that Negroes were nowhere in the United States accepted as full citizens and voters at the Founding. This is flatly untrue. At least five states, if memory serves, made no racial distinctions in their voting requirements.
One of them being NC(!), which did not disenfranchise free men of color till it revised its Constitution in 1830s.