Concerning the issue you ask about in The Great Gatsby, arguing absolutely, either way, is difficult. The novel, like most high quality, sophisticated fiction, is ambiguous. The book isn't a good guy and bad guy kind of book. Characters, like actual people, are mixtures of good and bad traits in the novel.
Tom tells Wilson that Gatsby owns the car that hits Myrtle. Wilson kills Gatsby. Tom is guilty. But it's not that simple.
Tom likely doesn't know that Daisy was actually driving the car. Daisy doesn't confess to him. Daisy guilty. But it's not that simple.
Gatsby plays a part in Daisy not telling Tom, and Gatsby certainly doesn't blame her. He lurks outside of her house all night in order to protect her from Tom. He still loves her. Is Gatsby guilty of his own murder?
One could go on and on in this way. You could argue that Wilson pulls the trigger, so he and he alone is responsible. Tom and Daisy certainly couldn't be charged with Gatsby's murder. They didn't put Wilson up to it.
And Gatsby's illusion is Gatsby's illusion. Is Daisy supposed to lie and pretend that she never loved Tom just to fulfill Gatsby's illusion? Gatsby tries to recapture a past that never was. He is on a quest that is doomed to fail. Yet, does that mean that he causes his own destruction?
In short, I suggest that your question is unanswerable. I suggest that to answer your question is to simplify an ambiguous novel that, in terms of its ambiguity, accurately reflects life.
But, I know you have an assignment to do, so, if I had to, I'd go with Gatsby causing his own downfall. Though he, like others in the novel, is a victim of Tom and Daisy, he is the foolish one who chases after a dream that never was. Daisy never loved him the way that he loves her. And when it comes down to it, he isn't satisfied with having Daisy love him now, in the book's present. She does love him and is ready to choose him. But that isn't enough. He has to maintain the illusion that their love is something special, that it is never ending, and that Daisy always loved him and never loved Tom. As Daisy tells him: he asks too much.
Gatsby causes his own downfall. There, I've perjured myself for you. Hope it helps!