Traditionally, for Christians, Good and evil are understood in terms of the 'unseen warfare' or 'spiritual warfare' between (one the one hand) God and the angels and (on the other) Satan and his demons - humans are the location of this warfare, rather than its instigators - and the main decision of each person is which side they will take.
At the same time, some people are mostly nice, while others are mostly nasty.
And there need not be much or any correlation between Good-nice and evil-nasty: indeed, the character type of Good-nasty is a favourite in literature.
A recent example of Good-nasty is Severus Snape - who is probably the most interesting and moving character in the Harry Potter series by JK Rowling.
Yet, the lack of a framework of spiritual warfare means that the secular perspective misinterprets Snape as a 'morally ambiguous character'.
Snape is not morally ambiguous - he is nasty, as is abundantly demonstrated throughout the series. However, like Harry, he has Rowling's prime twin virtues of Courage and self-sacrificing Love - and these mean that he has chosen the side of Good, as it emerges near the very end of the seven volumes.
Throughout the series we should not be in doubt that Snape is nasty - what we are unsure about - and what the novels mislead us about - is the answer to the question: What side is Snape on ?
In the Harry Potter series, the background of unseen warfare between God and the Devil is only hinted at - and superficially replaced (for much of the series) by Dumbledore versus Voldemort: nice versus nasty, kind versus cruel.
Yet in the Deathly Hallows we recognize that Dumbledore is sometimes nasty, has several nasty traits, has a history of cruelty.
At the secular level, this subverts the moral clarity of the novels - but not if a backdrop of spiritual conflict is acknowledged.
The real conflict in Harry Potter, and in Lord of the Rings and in the Narnia books, is between God and evil - and the bottom line evil is not cruelty but heresy: evil wants to be worshiped as God.
This is clearer in Tolkien and Lewis - but there are sufficient hints in Rowling that this is 'The Dark Lord's ultimate purpose: to be worshiped as an immortal god. Voldemort is, of course, cruel - but it is his indifference to others (e.g. killing Cedric Diggory as 'the spare'), his treating them merely as a means to his end, which is portrayed as his worst sin. They exist only to serve and worship him.
My point would be much clearer is there was a nice but evil character to mirror Snape as nasty but Good - however, I don't think there are any.
Indeed, this is a major deficiency in most modern/ recent literature and narrative art: the way that nastiness and evil are conflated.
Evil people in books, movies, on TV always turn-out to be nasty (cruel, sadistic etc) whereas in real life many of most devoted the servants of evil are (mostly) nice, and not especially cruel.
Can anyone come up with a good example of a genuine, clear cut nice-evil character from art?
Somebody really nice, and really evil?