Well first you have to ask yourself: Do I think Shylock is portrayed as a victim or as a villian? The answer to that question is the first part of your thesis statement.
Second you have to ask yourself: Why do I think that and what evidence do I have to support such a claim? The answer to that question is the second part of your thesis statement.
So, a simplistic way of presenting a thesis here would be:
Shylock is portrayed as a _______ in the society he lives in because of _______, _______, and ______.
From there you can make your body paragraphs about each of the main reasons you have for thinking he is a victim or a villian and the evidence you have to support these main reasons. You will probably want to include and analyze textual evidence from the play to support specific claims that you make as well.
Show us what you have after that. Or better yet try just writing the intro paragraph (include your thesis statement and a sentence or two on the main ideas of each body paragraph as well as a strong opening hook should you deem it neccessary) and we can take a look at it.
Tyler