What is Macbeth's evil side associated with?

There is much debate among Shakespearean scholars or general readers of literature as to what is the root of Macbeth's evil side. Some attribute it to the dark influence of his wife while others blame Macbeth for his own actions. Basically, Macbeth is a tale detailing the potential of man for downfall, and how greed/ambition can darken even the most stout of characters.