If you mean that evil is a perception that we all agree on as the ultimate abhorrent behavior possible, I understand but I don't necessarily agree. We won't all have the same perceptions.
The systematic killing of people as happened in the holocaust. That could fit everyone's perception. These people were seemingly sane and rational people in a way. They can't have all been criminally insane or driven by a psychosis they had no control over.
The other example you gave...I'm not so sure. I have to admit to not sharing the general public's obsession with that case so I don't know all the officially released details much less what actually happened.
9/11 was not what I'd consider evil. Tragic and despicable yes. Evil no. Middle Eastern terrorists see the US and other western governments as enemies. They see their own governments as the enemy. They have some good reasons for that view.
Was the US government systematic eradication and subjugation of the tribes evil?
Nevertheless I suspect that despite semantics, deep down we all think of "evil" in biblical terms.