Bushy, slanted eyebrows. Pointed goatee. We sometimes see these as diabolical. And as it turns out, it has everything to do with the geometry of evil.
Dr Derrick Watson and Dr Elisabeth Blagrove of the Unversity of Warwick think that those features give a face a shape more like a downward pointing triangle, which people seem to find threatening.
Volunteers in a study were asked to look at pictures of positive, neutral, and negative faces. They detected the angry faces first. This makes sense. If someone near you is troubled, you want to look them over as quickly as possible to make sure everything's okay.
The volunteers were then asked to look at pictures of triangles, pointing up, left, down and to the right. The subjects looked at the downward facing triangles as quickly as they did with the negative faces before. The shape reminded people of an angry or negative face. Which in real life and in faces involved a sharp chin, or a goatee if you must that makes the face come with a sharper chin.
As to why people tend to see this as threatening, is unknown just yet. Maybe its because of TV and movies. Not forgetting cartoons too.