Good Cannot Comprehend Evil

Bob and Alice are being terrorized by Doctor Deathmurderkill. He has hunted them, attacked them, and nearly gotten them killed on several occasions. Bob is convinced that Dr. DMK has kicked the dog more than enough times to warrant fighting back with lethal force. Alice, however, insists that he must have a good reason, or maybe that it's all just a big misunderstanding. Despite any evidence to the contrary, Alice refuses to believe that he can actually be 'evil'. After all, no one's that heartless, right?

"So that was Mrs. Lundegaard on the floor in there. And I guess that was your accomplice in the wood chipper. And those three people in Brainerd. And for what? For a little bit of money. There's more to life than a little money, you know. Don't you know that? And here ya are, and it's a beautiful day. Well, I just don't understand it."

—Marge to Grimsrud, Fargo

This trope is, in its essence, a character who firmly believes they're in a world of White and Gray Morality, that everyone can be redeemed, and there is no Moral Event Horizon. Whether Alice is right or not depends on the work's placement on the Sliding Scale of Idealism vs. Cynicism. In an idealistic show, this attitude could very possibly lead to a Heel Face Turn and a Defeat Means Friendship. In a cynical one, expect her death to be the very thing to push the villain over the Moral Event Horizon.

While not Always Female, this trope is usually limited to the very naive and very young, or the very old and war-weary, who've grown tired of the "black and white, us vs. them" attitudes of those around them.

Note: Be careful not to just stick this trope onto any pacifistic characters, whether technical, actual, or martial. The tropes are related, but one does not automatically imply the other.

Compare Stupid Good, The Pollyanna, White and Gray Morality. Contrast Evil Cannot Comprehend Good, Complete Monster, This Is Unforgivable!.

Examples of Good Cannot Comprehend Evil include:

Film

Star Wars: The Jedi Order suffers from this problem, and pay dearly for it, as the prequels show. In fact, the Star Wars Expanded Universe has shown again and again that for a group that acts as champions of good, the Jedi Order has absolutely no understanding of the Sith. Even when they're starting to turn into Sith.
Marge expresses this in Fargo, as seen in the page quote.
Most characters in No Country for Old Men also struggle with this, usually when they face Anton Chigurh. Likewise, Anton Chigurh cannot understand why his victims always implore him to have mercy.

Literature

Harry Potter: Dumbledore has shown that he can understand quite a bit about Complete Monster Lord Voldemort. However, it turns out that Dumbledore was unable to figure out that Voldemort hid one of his Horcruxes in the Room of Requirement. Why? Because Dumbledore was a model student who never cheated and hence had no need to use the room. Harry, however, was certainly not a model student, he cheated a couple of times, and he used that room, so he could figure it out.
In Death: Dr. Mira in Midnight In Death is unable to find out why David Palmer is such a Complete Monster. Eve Dallas is unable to find out why her own mother is a Complete Monster in New York To Dallas.
In The Silmarillion, it is said that the Valar don't understand Morgoth's evil, and explicitly didn't understand it was incurable.
Sisterhood series by Fern Michaels: The Vigilantes have shown that they cannot understand why people like the Monarch HMO in Payback, Karl Woodley in The Jury, and Maxwell Zenowicz in Fast Track are such evil people.
This is a recurring theme throughout No Country for Old Men.
Early in the Discworld series, Granny Weatherwax of all people. The Duchess mocked Granny's attempt to make her have a Heel Realization, claiming that Granny was naive for believing that all people really are good deep down.
In the Warrior Cats series, when Bluestar's suffering from dementia and is absolutely convinced that WindClan is stealing prey, she won't listen to Fireheart when he gives her evidence that a dog has been killing the prey. She tells him that he's a good and noble warrior, so he can't comprehend that other cats would have morals any less pristine than his own. He especially thinks this comment is odd, since he was the one that exposed Tigerclaw as a traitor and murderer.

Live Action TV

In the Star Trek: The Original Series episode "The Savage Curtain", Surak, Spock and President Lincoln have a hard time understanding the motives and actions of the opposing "evil" side. Only Kirk seems to have a grasp of their potential for deceptiveness and duplicity.
In Power Rangers Wild Force, Cole starts out as this, refusing to believe any creature is inherently evil and hoping to find a peaceful solution to the conflict with the Orgs. Sadly, his assumption that Orgs have anything in common with predatory animals hunting to feed themselves proves wrong - they are heartless beings and are literally Made of Evil. Fortunately he catches on fast, but he still refuses to find any pleasure in fighting them or reacting to their leader out of anger or revenge even after discovering he is the one who killed his parents.

Video Games

In Mass Effect 3 a defector from an evil organization thought that she had hidden families safely on a remote world, because she "couldn't imagine" the organization's leader coming after them. Shepard says "That's what evil counts on..."
- After discovering the various atrocities at Sanctuary, Ash (who is by no means a naive character,) says she truly cannot understand what motivated it. She also says that she's actually glad she can't, as it makes her feel human.

Western Animation

The Batman has one episode where Batman actually tries to make sense of The Joker. By the end, it is safe to say that Batman figures out that doing this is an exercise of futility and madness.
Aang holds this view for the majority of the first season, in keeping with his pacifistic views. Even at the end, when he's facing a Complete Monster like Ozai, he can't bring himself to kill him until the lion-turtle gives him an Eleventh-Hour Superpower.
Henry Pim in The Avengers: Earth's Mightiest Heroes will often try to reason with enemies first, especially if they used to be his villainy-rehab patients. In his defense, it almost works on Wonder Man before Iron Man brings down an Interrupted Cooldown Hug.
Samurai Jack: Up to Season 5, everything Jack has seen regarding Good and Evil has been black and white, and while he clearly believes humans have free will, he overestimates the power of such a concept. The idea that anyone could be indoctrinated from birth to have outstanding virtues like loyalty and dedication while simultaneously being psychotic killing machines (as the Daughters of Aku clearly have been) is new to him, and until he actually manages to talk to Ashi, he's almost as bad at understanding how it works as she is.

Real Life

Ulysses S. Grant's presidency was one of the most corrupt in American history. Historians often attribute this to Grant's political naivete - he himself was so morally upright that he couldn't see the shady dealings around him.
- Or maybe he's not upright, he's just too arrogant to know that he shouldn't trust so many people?
Churchill assumed that Hitler's orders to "stand and die" were to buy time to complete the (nonexistent) Alpine Fortress. He was Hitler of course, but even he wouldn't order people to die for no practical reason would he?
Livia Bitton-Jackson, a Jewish immigrant from Europe got a job teaching in America. One of her students asked, "What is that number on your arm."