Article 4, Section 1 (A4.S1 for short):
"Each State to Honor all Others. Full Faith and Credit shall be given in each State to the public Acts, Records, and judicial Proceedings of every other State. And the Congress may by general Laws prescribe the Manner in which such Acts, Records and Proceedings shall be proved, and the Effect thereof."
In other words, A4.S1 says, according to Annenberg Classroom
"Article IV, Section 1 ensures that states respect and honor the state laws and court orders of other states, even when their own laws are different."
The Supreme Court decision decided on Dred Scott v. Sandford (1857), according to the Britannica
"…ruled (7–2) that a slave (Dred Scott) who had resided in a free state and territory (where slavery was prohibited) was not thereby entitled to his freedom; that African Americans were not and could never be citizens of the United States; and that the Missouri Compromise (1820), which had declared free all territories west of Missouri and north of latitude 36°30′, was unconstitutional."
Dred Scott and his owner moved to Illinois and Wisconsin, both free Northern states, meaning, slavery was prohibited in these states. If this was the case, even if black men and women could not be citizens, they have to legally be free, right? Since not only was it federal law declaring these states as free (Missouri Compromise of 1820), but also the states themselves, at least Illinois, were greatly pro-abolition. By saying that just because he was in a free state, he is still a slave, is this not defying A4.S1?
Source: reddit post