Look at the setting and the character's state of mind, and then think about what you want the reader to see. Is there a descriptive symbol or two that works naturally within the scene to help foreshadow an event or theme, or create insight into the character's emotional plight?
** Please note the symbols listed here are either old world beliefs & superstitions or iconic symbols adopted by film and media. None are meant as slurs against any individual or group.
Animals that kill for sport, not need
Foul weather (sea storms, etc)
Crimes against children
The color black
Nuclear mushroom cloud
These are just a few examples of things one might associate with Evil. Some are more powerful than others. Three sixes grouped together is a symbol that is immediately recognized by many, and likely will not require reinforcement. However, storm clouds on the horizon may not foreshadow evil on its own. Let the story's tone decide if one strong symbol or several smaller ones work the best.
**Also, with this entry in particular, consider the beliefs of your characters. A practitioner of Wicca would not consider witchcraft to be evil, nor necessarily would a society where blood rites are common practice. What is or is not evil will be always be in the eye of the beholder.