The correct answer is Northerners.
The Missouri Compromise allowed the state of Missouri to enter the US as a slave state. Along with this, it allowed Maine to become a free state and established the 36'30 line as a means to determine where slavery will exist in terms of adding new territories to the US.
Northerners were angry at this because any new slave state added would result in more pro slavery politicians in both branches of Congress, including the House of Representatives and the Senate.