What made the 1857 Scott v. Sanford (Dred Scott) decision so controversial

He ruled that Congress had no power to limit slavery in the territories.
In Dred Scott v. Sandford (argued 1856 -- decided 1857), the Supreme Court ruled that Americans of African descent, whether free or slave, were not American citizens and could not sue in federal court. The Court also ruled that Congress lacked power to ban slavery in the U.S. territories.

Both countries had to aprove annexation.