Draw some parallel lines on the face to see if everything aligns. The eyebrows, eyes, nose and mouth aren't properly aligned at the moment. His left eye is closer to the nose than the right eye and the eyes themselves are flat. Think how the eyes are inside the skull and the visible part has eyelids above and below. There's nothing under the eyes, so give him some cheekbone. Horns would work better closer together and the other one pointing almost at the viewer to give it some depth. Enlarge the canvas so you can draw the jaw and the neck as well, just for the sake of practice.
Study what makes an expression evil and give him some of those features.
The lower lip could be fuller and upper lip more defined.
Put him in an envirenment suitable of his demonic nature.
I can scetch you something if there are parts you find really difficult.