The play deals with some controversial topics--murder, the supernatural, the evil side of human nature. I have always found the animal imagery in the play very intriguing. Macbeth references animals which typically are not warm, fuzzy, or cuddly...snakes, scorpians, tiger, rhinoceros, bears, etc. Those who are good in the play reference animals with more positive connotations. This symbolizes the very nature of humans and of the world...the night Duncan is murdered horses eat each other, owls screech unnaturally, etc.
The darkness of the play even warrants precautions taken by those who perform it. Most actors are forbidden to say the name of the play inside the theatre for fear of the evil attached to it causing some malfunction in the performance.