Why are demons portrayed as monsters?

The ultimate demon, the devil (Satan), named Lucifer, was once an angel of light. He got jealous of God's power, and asked God to give him more power than the other angels. When God refused to give it to him, Lucifer 'fell from grace', and became the demonic Devil. The jealousy and spite is represented in Satan by having an horrendous expression and a feral appearance.