Complex sentence: The notions that Nebe's motivations were anything other than Nazi ideology have since been discredited by historians who describe him as an opportunist and an "energetic", "enthusiastic" and "notorious" mass murderer driven by racism and careerism.
Split into simpler sentences: The notions that Nebe's motivations were anything other than Nazi ideology have since been discredited by historians. They have described him as an opportunist and an "energetic", "enthusiastic" and "notorious" mass murderer driven by racism and careerism.