Great article Dan! I think the idea of evil can only be applied to humans, because the definition of evil can be seen as "actions taken in a way to produce suffering where suffering is unnecessary".
You're completely right that we may perceive of a wolf ripping apart a rabbit as "savage" or "evil", when in reality it is simply following it's nature to eat and survive.
While it's hard for us to know if the wolf is thinking "I'd like to kill the rabbit slowly so that I can savor the terror and pain it experiences" , it is most likely acting according to it's biological impulses and drives of hunger.
The fact that humans can consciously decide to enact more suffering than is necessary (torture rather than a quick death) is what allows us to be categorized as evil. A quick skim of history (even as recently as the 20th century) will give us no shortage of human atrocities (Holocaust, Rape of Nanking, Forced famines & concentration camps in Russia).
I agree that it is not productive to identify people as "evil" to explain away the incomprehensible and monstrous acts they inflict upon other people. We need to be more sophisticated than that in order to tackle the problem.
The lesson that we are supposed to learn by studying the evil acts of people in history is not that specific groups of people (Nazis, Japanese, Russians) are capable of engaging in horrific and inhumane acts, but it is within each individual that such capacity for evil exists and must be overcome.
“The line separating good and evil passes not through states, nor between classes, nor between political parties either -- but right through every human heart -- and through all human hearts. This line shifts. Inside us, it oscillates with the years. And even within hearts overwhelmed by evil, one small bridgehead of good is retained”
- Alexander Solzhenitsyn
The last line of your post shows that you understand this message and I'm happy that you've decided to share this message with us. Thank you for writing this!