No one person can say that their opinoion on the existence of evil is correct, because an opinion is nothing more than what YOU think. Opinions change like the wind on a daily basis. For example: You want to buy a CD by Busta Rhymes. Your friend says, "Aw, that n***a's whack! I don't like him!" Well, you buy the disc anyway, because your opinion matters to you. Your friend comes over and you're listenin' to Busta. "This s***'s good!" Your friend says, while you're mad at the wasted $12 you spent because when you got to track seven, you said, "This n***a's whack!" LOL. Y'see what I'm getting at?
You can't base the existence of something soley on opinion, due to the fact that tomorrow, your view may be different.