In my opinion, you don't need to work with "darkness" and these kind of stuff that people like to call representing the 'evil'. Everything can be corrupted, the good can turn on evil. We use the quote "There are no such things as absolute good and evil. Rather, there are many subtle shades of gray".
Of course when you look for 'evil spirits, demons or whatever you call them or that", you'll find what you want. But, remember, working with revenge can turn you mad, sometimes in magic, you need to work with a high emotional status and this can affect you. You should look for entities that are named and known these ones who brings the justice (what is completely different of revenge, right?)