Demons and angels, black and white, we use these labels to distinctly describe an inherent contrast, based on an easily understood ideas. They may be the easiest way to get an idea to its simplest form, but it usually ends up being a big generalization of character. However, loose interpretations are what works for a world where time is limited, and surface level explanations are "good enough" for most people.
And when things come down to black and white, you tend to see a lot of contrast, with no real room for the grey to blend in -- that is were a lot of stereotypes form in this two sided issue. However, things in the real world aren't that simple, and an anime that has this type of theme running through it nicely, is Hataraku Maou-sama.
Black
As they say, the devil is in the details, and this devil -- named Maou Sadao -- doesn't have the personality to be evil for no reason.
If you have ever read any kind of fairy tail, where there is some kind of demon and hero conflict, you would think the Demon King -- Satan if you will -- would be someone truly evil for no good reason. However, Hataraku Maou-sama breaks that misconception, in favor of a more believable villain turned hero.
With these types of fairy tail archetypes being flipped on their head, I find it interesting to see most of the characters change as they begin to see Sadao's intentions being reveal -- just to live peacefully in Japan. Rather than cause destruction, he chooses to keep the land as untouched as possible, but that peace will be shaken soon enough, however not by someone you would expect.
White
Sarue Mitsuki was first introduced as the new competition across the street to MgRonald, the Sentucky Fried Chicken manager. However later on we find out he is an angel who has come to take back Emi's holy sword.
His real name is archangel Sariel, but don't be fooled by the fact that he is an angel. Sariel -- based on his action alone -- can be attributed as the villain of the series. Initially, he had most of the "good people" on his side, but soon turned on him, because of his harsh treatment to most people, and the disregard for the world he is currently in.
Grey
Hataraku Maou-same isn't that complicated of a plot, it points out that black and white comparisons aren't always going to be correct, by switching the roles of demons and angels. However, with that simplicity, we do need a wide range of anime, to display these facts more clearly to multiple demographic of people. If you do want a more complicated anime, that does deal with similar issues, I would recommend Shinsekai Yori.
Flipping around the stereotypically roles of demon and hero is certainly an interesting take on the topic, and I'm glad I didn't skip this one, because of the previous season's Maoyuu Maou Yuusha -- that ended abruptly and kind left me bitter on stories starring demons. But thats besides the point, Hataraku Maou-sama is a good light hearted story that I never really got to talked about that much last season. It covered the subject in question pretty well, in a comedic way, but also with purpose, which is good in my book.