You could make a case for Doc Savage being the first "pure" super-hero in the sense of his mission. Robin Hood and Zorro and the Scarlet Pimpernel had a specific local cause, fighting one tyrant or tyrannical regime. The Shadow fought crime. But Doc explicitly said his life work was to travel the world helping people in trouble and punishing wrongdoers. He was "a modern Galahad."
His idea of rehabilitation was as questionable (or downright wrong) as the Golden Age Wonder Woman. He kept it secret, too, thinking correctly there would a public outrage if what he did was revealed. I think if Wonder Woman's quasi-brainwashing of her captives to make them submissive and docile was made public, there was also be a huge uproar from the public. It's like A CLOCKWORK ORANGE.