Set up an essay plan (important to review how to structure an essay and the PEEL format). It can be helpful to create two mind maps at first, one for ideas as to how he is not responsible and another for how he may be responsible for the tragedy. You can then slot these ideas into an essay plan. For example, one theme you could pull out is Eddie's Eros for Catherine - you could then find three key moments in the play in which these are shown and complete step 2.
Find at least 20 relevant quotes you want to include. Using the previous example, a relevant quote could be when Alfieri advises Eddie to 'let go' of Catherine because 'there's too much love and it goes where it musn't'. Note how I have intergrated the quote into my sentence; this is a technique that will get you points!
Now that you have a plan and evidence to support your points, go ahead and start writing the essay!