Evil is, as so many things, a matter of perspective.
Some things are objectively impossible to reconcile with a solid moral standard - like serial killers, or brutal rape, or sadistic acts of torture - but most other things fall into a murky, hard-to-define dark grey zone.
Most people who do things, do them because they feel like it's the right thing to do. I don't think anyone wakes up in the morning and goes "Mwahaha, today I'm going to be eeeevilllll! >:D". They wake up thinking "Today I'm going to do this thing, because it's the thing that needs to be done." That's the kind of perspective I try to employ when I write antagonist-characters; they are the center of their own story, and believe what they're doing is right.
Desperate people do desperate things, and sometimes those things hurt other people. If someone is desperate enough, or convinced enough of the justice of their own path, hurting other people becomes an unfortunate consequence - or even a justified revenge.
I have very little interest in the stereotypical, evil-because-they're-EVIL, maniacal Dark Lord-type of villain - because there's no tension there. I never have to ask why the protagonists are fighting - they're fighting because their opponent is evil. The outcome is nearly always a foregone conclusion.
Complex characters springing out of a complex society are inherently more interesting to me.
