There're aren't evil people in reality either. "Evil" and "good" are simplification labels we like to stick on things to make decision makimg shortcuts, because no one got time to psychoanalyse random humans.
Also, just because your antagonist or "bad guy" has a reason of why they do the thinfs they do or believe the things they believe, doesn't mean your protagonist has to care about it. That stuff is mostly for the reader. Protagonists are people too, liable to use the "good" & "evil" labels and not give it a secomd thought, just like any of us.