Dred Scott, himself, thought that he should be free after being
a resident of a free state, Illinois, which was created under the
anti-slavery conditions of the Northwest Ordinance. Scott also
lived for an extended period at Ft. Snelling, in the Wisconsin
Territories, federal land that also prohibited slavery.
Abolitionists, most of whom were in the northern, midwestern, and eastern parts of the country, believed all slaves should be emancipated.
At least four of the judges who adjudicated the Dred Scott v. Sanford case (the District Court judge, one of the three appeals court judges, and two US Supreme Court justices) and the jury at the original trial agreed not just Scott, but his entire family, were already legally free, under the "once free, always free" doctrine many courts respected at the time.
Unfortunately, prior to the Civil War, those who believed in slavery held more power in government than those who wished to abolish the institution.