Magatsu Wanashi no Yuusha kari

Echizen is a game designer being ground under the boot of his high-pressure job. His demanding workplace won’t even let him do what he’s best at: building brutal puzzles and traps for players. When he quits his job, something suddenly sucks him into a hellish dungeon in another world. His path collides with the alluring demon princess Sophilia, but before she can explain how or why she’s summoned him, they’re threatened by a party of marauding heroes–and Echizen watches in horror as the “heroes” enact a genocidal massacre on Sophilia’s demon friends. With his life on the line, Echizen uses the world’s magic to put his game developer skills to use in Sophilia’s defense. The deadly traps he creates give the heroes some brutal comeuppance, but the rush of power unlocks Echizen’s most sadistic impulses…leading him down a path he may never be able to turn back from.
(Source: Seven Seas Entertainment)