Well according to the bible (which was probably already said) angels are like robots with no will of their own. They have no other choice but to serve God. But there were a few exceptions. Before the creation of man, Lucifer (a.k.a. the devil), rebelled against God. He was to be the most powerful and the most beautiful angel in heaven. So he convinced about one third of all the angels to follow him. God got angry and banished them. So according to the bible the devil is an angel, not some evil demon with horns as many say he is.