A sadist and a serial killer get married and go on a hellish (for everyone else) cross-country road trip from Hell itself. Then something comes from Hell (the one the couple made for themselves, not actual Hell), and the consequences of this couple enabling that being is a story for another game...which the dev has made and kindly linked to in the game's description. You should go check out!
This story manages to strike a good balance between the messed-up actions of the protagonists while injecting a bit of dark, self-deprecating humor to make proceedings not a glorification of death.