Arnim Zola

Credit to @D-Skelton

The mastermind behind HYDRA's scientific breakthroughs, and the man who harnessed the power of the Tesseract into a weapon, Zola was a brilliant man. He was captured after the war, and briefly put on trial before being deemed "unfit". The truth however, was an agreement with the US, top secret. He was saved in exchange for his service to SHIELD, an agreement that would one day come back to bite. He may have died in the 1970s, but his impact lives on, rooted deep within SHIELD.