Hi,
This is a play I know fairly well but my take on it can be a little different to mainstream. MacBeth is a play that over it's performance history has had several clear fashions of interpretation (The New Cambridge Shakespeare has a good account of performance history) and one recent (last 100 years) trend has been the increased focus on Lady MacBeth and her 'evilness'. I don't see this myself - try to read the text as if you have never heard of the story before. The interpretation history of this play would be a great essay topic in itself. Here are a couple themes I focused on while directing a product a few years ago.
1) The destructive side of ambition. The Witches in my production took on the role of ambition, directly ruining the lives of Banquo and MacBeth. I felt that Banquo was personally being more tempted by the images placed before him by the witches but was slower to act (his 'stars, hide your fires...' speach). MacBeth's ambition was high, but his nature was not to take a wrong step or going down a path when the ends justified the means. For ambition to be king, in the modern world substitute wealth, political/personal power and promotions and look at how these can compromise behaviours as a way of making this play relevant today.
2) The motivations of Lady MacBeth. It is easy to paint her as the cold-blooded killer but I feel this is a rather fashionable interpretation and a sterotype of stronger women that doesn't work for me. My belief is that she loves her husband dearly and understands him perhaps better than he does himself. She knows his ambition and his desire to be king but that he will balk at the step of action. She does not pursue power herself but helps her husband achieve what he wants. Immediately after the murders she wants this path to stop (about the time she learns the grooms are killed this starts) and starts to realise she underestimated the effect the murder would have on MacBeth.
Hope this helps - good luck with the essay.
Rus