Complex sentence: Darrow was stunned by Steffens' report that the brothers had admitted their guilt to him, but with his health worsening and his pessimism about the defense growing, Darrow agreed to permit the McNamaras to cooperate with Steffens.
Split into simpler sentences: Darrow was stunned by Steffens' report that the brothers had admitted their guilt. But with his health worsening and his pessimism about the defense growing, Darrow agreed to permit the McNamaras to cooperate with Steffens.