It would be difficult to ignore the overwhelming evidence that there is evil in the world. The real question is its source. Many view humanity’s anger and violence as simply “human nature”, without much thought as to a deeper cause. Christians assign the existence of evil into two broad categories, one Supernatural, and one in the Natural Realm.
I would love to read an existential, secular argument for the existence of Satan and his demons without using the Bible as one’s only proof text. Acknowledging there is sin in the world, or that caricatures of Satan and evil are ubiquitous throughout other religions, or people are demon-possessed, or houses are haunted, etc., for me, is not proof of the supernatural existence of a real Satan and his ilk. My reliance is on the Bible and, for those who reject it, I sadly can offer no other supporting evidence beyond the reality of evil in the world.