Never. Because who decides? Where does it stop? When you burn books you are trying to silence thoughts and ideas and even when those thoughts and ideas are objectively evil we need to understand them if only to stop them propagating through vulnerable populations that are susceptible to misinformation and lies. The answer is to teach critical thinking, not turn certain thoughts into a martyr or a disease whose existence then flourishes, hidden in the dark corners until the infection can no longer be treated without killing the patient.