Evil exists because of the existence of a totally evil being: Satan, the devil who is totally and irredeemably evil. In the Bible he is described as the deceiver, the dragon, the father of lies, our adversary, the murderer from the beginning. Everything he does is evil; he is implacably opposed to all that is good and virtuous.
The whole world lies under the control of Satan (1 John 5:19). Satan is not all-powerful, but he is very powerful, very devious, and tempts us to do evil things. This is why Jesus taught us to pray, “And do not lead us into temptation, but deliver us from evil (the evil one).”