I define "evil" within the Dresdenverse as being someone who has a complete and gleeful disregard or hatred of human lives.
All human lives, or just some? If it’s just some, then I’m reminded of things like drone bombing areas in the middle east where innocent men, women and children are blown to bits on a regular basis, the fire bombing Dresden (the city) and so on. Warfare is full of purposeful civilian deaths. Are those things evil? Does glee have to be involved? Is disregard and dehumanising not enough?
And, only humans? What if someone had disregard for dogs and just went around killing them as much as possible? How about someone who make fearie skin sofas to sit on while drinking champagne from the tiny skulls? Are they evil? How about a person who enjoys killing animals at a slaughterhouse, or do we become utilitarian here because it’s approved?
Not trying to be difficult here, but the term “evil” gets used a lot and I never really know what it means to the person who used it as it’s not describing objective reality, but rather a perspective and everyone has a different perspective (even the same person has different perspectives from moment to moment depending on any number of variables).
To answer the question based on this definition, though, I’m not sure we can give a real assessment. The book is first person and limited to only what Dresden thinks others are thinking. If I were to guess which Dresden thinks is the worst, then I’d say the genoskwa or whatever it was called based on his reactions to and feelings about it.
Objectively, I still think the outsiders are the obvious choice for the most potential to be harmful with Dresden a possible second (depending on this “starborn” thingie).