It's been ten years since the demons have taken over Earth. Since then, humans have been treated like trash. A righteous faerie, who cannot stand seeing humans being trodden upon, came to Earth from Heaven. She is searching for a kind-hearted person to help her fight the demons, because faeries cannot attack demons directly due to a pact from the ancient times.

From MangaHelpers:

In the past, the three realms of the Underworld, the Earth, and the Heavens were separate. Demons lived in the Underworld, humans lived on Earth, and the fairies lived in the Heavens. However, ten years ago, the demons invaded the Earth. Now they rule the Earth, and all of humanity are their slaves. They still have jobs, attend school, and so forth, but they all live in fear. If a human offends a demon, even by accident or by doing nothing at all, they can't stop the demon from killing them. One special human girl, Osoreyama Miyako, sees the state of the world, sees the path her pitiful life will follow, and decides to end it all.

The humans aren't the only ones concerned with the demon takeover. The fairy Jin has been sent to right the balance of the three realms. However, by old treaty, she can't fight demons directly. She must make some human into a magic girl to fight in the defense of humanity. The naive, idealistic fairy chooses Miyako to be her magic girl. On the very night Miyako plans to take her own life, Jin forms the contract and dubs her "Belial Strawberry." To Jin's horror, though, Miyako does not have a heart full of the love of justice. Her heart is full of anger, bitterness, and despair. When Jin makes her a magic girl, all Miyako hears is that now she will have the power to slaughter her oppressors. After she brutally murders a demon in public, in the defense of a helpless cat, the newspapers name her "Slaughter Magic Girl Belial Strawberry," and her story begins! [tethysdust]