Othello and Victor Frankenstein are presented in the beginning of their respective works as heroes because they make great gains on behalf of humanity and contain great moral ethics.
In Othello, The Moor remains steadfast in his belief of his ever faithful seemingly devoted friend, Iago. As audience members we know Iago is up to no good, but a good hero generally believes in his subjects as Othello does. Othello is likewise expected to go out to war and defeat the Turks because no one else can do it like he does. He must be a mighty and wise general otherwise the government would have left the situation up to Montano and not bothered Othello seeing that he had just been married.
In Frankenstein, Victor (in painting a picture of himself throughout the first few chapters) demonstrates his desire to do something great for mankind. He has a passion for science and specifically the ability to prolong life or preserve it. The picture of his family and education demonstrate a morally strong background.
For each of these characters, the beginning of their tales paint them as heroes because they are succeeding and they seek to do good for humanity or at least the people they represent. However, both seem to become tragic heroes as time goes on because we do see downfall in each of them. Perhaps that is almost more heroic to the reading audience because we can learn from their pain and the tragedies that did ensue.