For a majority of northerners, the most outrageous part of the Supreme Court's ruling in the Dred Scott case was

that Congress had never had the power to prohibit slavery in any territory.

Explanation:

The Dred Scott Case against Sandford (also known as The Dred Scott Case) was a crucial lawsuit in the history of the United States, resolved by the Supreme Court of that country in 1857 in a decision that prohibited any inhabitant of African descent, whether they were slaves or not, the right to citizenship and that took away from Congress the authority to ban slavery in the country's federal territories.

that Congress did not have the power to prohibit slavery in any territory.

Explanation:

In Dred Scott v. Sandford, the Supreme Court ruled that Americans who were African descendents, regardless if they were free or slave, were not considered to be American citizens and, as a result, they could not file a lawsuit in federal courts. The Court also ruled that Congress was not empowered to ban slavery in the territories of the United States. Finally, the Court decided that living in a free state, even for many years, did not free slaves, and declared the Missouri Compromise unconstitutional.

Dred Scott was a black slave who filed a lawsuit against Sandford so as to get his freedom. He argued that as he had lived in a free state, he was also a free man.

Northerners were against slavery. By means of this ruling, the Court benefited the southern states.