I am writing a book. However, I'm having trouble creating an antagonist that seems logical, or in other words, one that people can empathize towards. I was going to ask this on writers stackexchange, but I think here is better, as this is more of a philosophical question. Because I can't create a believable antagonist, I'm also having trouble creating conflict and plot. So here's my question: How do people justify doing bad things, and how do others empathize towards them? Thanks for any advice!
It seems to me that your question has more to do with moral uncertainty than with moral virtue, or lack thereof.
A believable antagonist is one towards which one feels ambivalent. This has no necessary relationship with evil per se, as a moral concept, or even as an element of practical philosophy. It has to do with the need we all have to deal with feelings of regret, self-doubt, internal contradiction.
In a way, the problem is not that we find it hard to justify bad deeds. It's that we find it too easy. A good "bad" character in a play, a novel, or a movie, is usually a profoundly human figure. That's what you should look into while creating it, I think.
The easiest way to be evil is to be absolutely certain what good is.
Whether good for you is family or cultural loyalty (mafias, honorable invaders and foreign agents), a given rigid creed (totalitarians and terrorists), the advancement of knowledge for its own sake (evil scientists), defense of the current order (cruel judges and abusive police), capitalist competition (heartless robber barons) or simply your own safety (colonial empires), it is not necessarily good for most of the rest of us.
The ease with which people find that kind of evil relatable is shown by the formulaic 'Saturday morning cartoon' villain, who would be a good guy, if he were less isolated and less certain.
Another flavor of the same kind of relatable evil is total lack of awareness that what one is doing is harmful. You may have no effective conscience (psychopaths, mindless beasts, those whose obsessions or addictions blind them, and overly empowered children), you may not believe in or be aware of those harmed (distant aliens and destroyers of fairylands everywhere), you may be thoroughly trained to see the protagonists as insignificant (slaveholders, 'more advanced' civilisations supplanting life-preserving cultures, brainwashed protocol drones, servants of misguided masters) etc.