Why do you think the director of Macbeth decided to create such violent scenes on screen?
Given that you did not provide the name of the actual production in question, the answer will refer to any productions of Shakespeare's Macbeth and the interpretation of the action by the director.
First and foremost, one must remember that Shakespeare did not write down any of his plays. Therefore, all stage directions were omitted from the manuscripts. While setting were briefly named, not mush more was given to any one who desired to produce the play.
Outside of that, one must remember that productions are all about making money. Some producers are simply more concerned with revenue than adhering to the original manuscripts. Many times, plays are simply "Hollywood-ized" in order to try to attract the most patrons and watchers of the movie.
For most, watching the 1971 BBC production of Macbeth would not be exciting. It is simply too dry and adheres to strictly to the actual play (not a bad thing for Shakespeare enthusiasts). Many would much rather see the contemporary re-telling of Macbeth (as produced by Sam Worthington). This movie is set in the ganglands of Melbourne and would most likely appeal to a modern audience.
Therefore, in short, most directors simply take the artistic freedoms allotted through personal interpretation and societies desire to see more blood and gore.