The Radicals passed the Tenure of Office law to prevent Johnson from replacing officers they had approved of. Johnson thought it was unconstitutional. Three days after Johnson fired Stanton as Secretary of War in violation of the Tenure of Office law, the House voted to impeach Johnson. The principal charge against him was the violation of the tenure law.
Johnson was right about the unconstitutionality of the tenure law. The Supreme Court of the time was intimidated by the radicals and wouldn't rule on the law, but the principles of the law were later ruled unconstitutional by the Supreme Court in 1926.
Radical Republicans were a dark blot on the nation's history. In Texas, they tried to retain power by force of arms after being voted out by the people. In Louisiana, they got a friendly judge to block elected members of the government from taking office, but let some who had not even stood for election fill those posts.
Here is a description of the views of Thaddeus Stevens after the war by CSA General Richard Taylor who met with him then:
He wanted no restoration of the Union under the Constitution, which he called a worthless bit of old parchment. The white people of the South ought never again to be trusted with power, for they would inevitably unite with the Northern "Copperheads" and control the Government. The only sound policy was to confiscate the lands and divide them among the negroes, to whom, sooner or later, suffrage must be given.