In the world of evil there seems to be a progression. In the beginning evil looked evil. Evil worked by fear. We are afraid of monsters and demons that look like monsters and demons. So evil progressed from monstrous to beautiful.
Why are demons portrayed as beautiful? Monsters want to take something from us. A beautiful monster will get us to give it to them freely. Beauty invokes desire. Desire, by nature, requires something worth having. The devil know this. He can be monstrous if need be. But he can also be beautiful if necessary.
"And no wonder! For Satan himself transforms himself into an angel of light." 2Cor. 11:14
Just because something is beautiful and desirable does not make it right to have or good for us. Satan did not tempt Eve by being monstrous. He beguiled her with smooth words and enticed her with a beautiful appearance. Some things never change. Desire has destroyed more souls than fear ever could.