Demon on the playground

Back in Elementary School one of my classmates told me about a nightmare he had that involved him and his sisters. My classmate Arlen told me he was walking to the playground where his two sisters were and he noticed there was this boy chatting with them. Arlen asked his sisters who he was. They replied he was new to the neighborhood and came to say hi, Arlen introduced himself when all of a sudden the boys' eyes turned crimson red, Arlen looked at his sisters, and both of their eyes turned red as well. Horns started growing out of their heads, and Arlen was terrified, he started praying out loud saying The Our Father when he got attacked by the boy or demon telling him to stop praying. Arlen kept on praying even though he was bleeding from his injuries when the demon got irritated even angrier and jumped at him.

I personally believe there is good and evil on this planet and we have freedom of choice, I believe there are evil forces that torment others but for what reason? To this day I don't know why evil tries to infiltrate our lives and dreams. What do you think the answer could be?