A fascinating goal. To be honest, you will have trouble proving that it has no right motives at all. The reason? The ending. The Macbeths pay for their crimes. They suffer and die. That's usually considered the ultimate punishment, and you'll have to answer that when you make an argument that it is completely dark.

Now, if you want to prove it contains evil, or shows evil, that's easy.
Start with each murder.
Add in the fact that most of them break explicit oaths of loyalty.
Add the fact that these disrupt the proper order of authority.
Add the lies.
Add the way that Macbeth gives in to ambition quickly.
Add the possibility that the witches are themselves evil.