Why did many republicans become angry after johnson took office?

Johnson retained all of Lincoln's cabinet officials. He at first followed a harsh policy toward the defeated Southerners, denying political rights to anyone who had supported the Confederacy in a military or governmental role during the rebellion. He also agreed to the arrest of several prominent Confederate officials. Johnson pursued this course at the urging of Secretary of War Edwin Stanton, an Ohioan, and Radical Republicans in the United States Congress.

The verticals lines in the liner