Well, it is important to remember that plays and movies are two very different mediums, so of course you need to make some changes to suit the differences. In a sense, to make a film from a play, you have to make your own interpretation of what the play contains, similar to the kind of choices that any director must make if they wish to stage the play. However, what is interesting about the film of Macbeth as directed by Roman Polanski is the focus on death and violence from the first. His version includes the death of the original Thane of Cawdor, as he chooses to jump off and hang himself. The action is accompanied by eerie music that dominates the descent of Macbeth into evil, and there is a lot of blood and gore in the way that Macduff's family is killed, Banquo is killed and then the murderers themselves, having killed Banquo, are themselves dispatched. Another key image in the play that is developed in the film is Macbeth's famous image from Act V scene 7, when he compares himself to a bear being baited:
They have tied me to a stake: I cannot fly,
But, bear-like, I must fight the course.
This image is developed by having an actual bear-baiting during the play, and then the grisly corpse of the bear being carried away, focusing on how Macbeth is, by the end of the play, pictured as a bear being baited, awaiting his death.
These are some of the major points of what I remember from the film, but I do recommend you watch it yourself as it is a very good, sinister interpretation of the text.