In the Bible it says that the Devil was once an angel. How is this possible if the Devil is so evil?
In answer to this question let me first say that I search through the scriptures and could not find a single scripture that says that the Devil was once an angel. And quite frankly, it surprised me. When I first read the question, I did not hesitate to assume that the statement was true--that the Devil was once an angel. In fact, I must confess that I have personally believed this to be true for several years. However, once I started searching for the scripture that specifically says that the Devil was once an angel, I could not find it. I did find in 2 Corinthians 11:14 that Satan can take the appearance of an angel, but that is not the same thing as Satan once being an angel. We read in Matthew 25:41 and Revelation 12:9 that the Devil has angels that serve him. But again, just because angels serve him does not mean that he was once an angel himself. Additionally, the word "angel" is really best translated as messenger. In other words, Matthew 25:41 and Revelation 12:9 could be talking about the Devil and his messengers as opposed to referring to what we commonly think of as angels.
Now perhaps I am overlooking something which the questioner has in mind and perhaps I did not do a thorough enough search, but it is my conclusion at this time that the Bible nowhere says that the Devil was once an angel. This, however, does not mean that I am not going to address the question, but rather that we should be very careful as to what we assume the Bible does or does not teach. I remember one time many years ago when I was trying to learn more about the sin of laziness I was looking for the scripture that says, "Idleness is the Devil's workshop." And while that principle is certainly true, you will not find that phrase within the Bible. Neither will you find the phrase, "Cleanliness is next to godliness." It just goes to show that sometimes we can be lead to think that certain things are taught within the Bible when they are not. This is why we need to be constantly studying it and making sure, like the Bereans, that the things which are taught are so (Acts 17:11).
However, let's rephrase the question as follows: the Bible says that God is the creator of all things (Ephesians 3:9). How could God create the Devil if he is so evil? Perhaps we should first ask, "Did God even create the Devil?" The scriptures appear to indicate this to be true, not just from the idea that God created all things, but more specifically that God did create the Devil. Job 1:6 says, "Now there was a day when the sons of God came to present themselves before the LORD, and Satan came also among them." The fact that Satan came "among" them indicates that he was of the same origin as these others who were called, "the sons of God." In other words, they were God's creation. So God originally created the Devil.
But did God create the Devil evil? There are a couple of other scriptures that we could look at to help us answer this question. God cannot create anything that is evil. In James 1:17 we read, "Every good gift and every perfect gift is from above, and cometh down from the Father of lights, with whom is no variableness, neither shadow of turning." So the Devil must have been good when God created him. So how did the Devil become evil? We find a hint of this answer in 2 Peter 2:4 and Jude 1:6. These passages suggest that some of God's angels were created good and then choose to do that which is wrong. Peter says that God did not spare the angels who sinned, but cast them into chains of darkness. Jude says, "And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day." Now while the Bible does not specifically say that Satan was an angel, it would not be difficult to understand, based upon the fact that some angels were created good and then sinned and fell, that Satan also was created good and then sinned and fell. However, unlike these angels that sinned and fell, the Bible indicates that Satan was the first one to do this. In John 8:44 Jesus says that the Devil is the "father" of lies. This indicates that he was the first one to do it. Since basically all evil is simply a modification of the truth into falsehood, this would indicate that the Devil was the first created being to sin.
So what is the Devil? Was he an angel or something else? Well, the Bible does indicate that there are other heavenly beings besides angels. Michael is called an archangel in Jude 9 indicating that there are certain positions of authority among the angels. We read of Cherubims some fifty-seven times in the Bible. These were the heavenly beings that Moses was to form on the Ark of the Covenant (Exodus 25:18). We also read in the Bible of Seraphims in Isaiah 6:2 and Isaiah 6:6. Ezekiel also describes some types of "living creatures" in Ezekiel 1 that appear to be some kind of heavenly creation of God. The basic problem is that we just don't know what God originally had intended for Satan. Deuteronomy 29:29 says, "The secret things belong unto the LORD our God: but those things which are revealed belong unto us and to our children for ever, that we may do all the words of this law." God does not answer all of our questions about Satan, but he does reveal that he is our enemy and that he doesn't want us to be saved. Let us be warned that Satan is going to try to destroy us and we should take heed to what the Bible does say about him.