Sono Yuusha, Utsuro ni Tsuki: Yuusha Wa Shoujo wo Sukuenai

In a world of sword and sorcery, the Kingdom of Nor, one of the four allied nations at war with the Demon Lord, attempts to defeat him by summoning a hero from the modern world with a secret spell. However, the hero who appears is a soulless serial-killer. The name of the "hero" is Kagei Soji. He makes decisions based on his own whims, without regard for reason. Immediately after the summoning, under the bright blue sky, he kills the princess and her guards, and escapes from the castle. All because he had decided to "kill if it was sunny, or let them live if it was cloudy." In the forest, Soji is rescued by a slave girl named Mythos, and they end up traveling together. The journey with Mythos is peaceful, but short-lived. For a powerful pursuer named "Mixing" is under orders to track down and kill the unruly hero...... Do not succumb to fate! A unique dark fantasy!

(Source: Dragon Novels, translated)