Two Bronies, through typical isekai means, end up in Equestria. A dream come true for most here, right? Except for one small caveat: Neither of them knows of the others existence. At least, not initially. Unaware of this fact, and believing the world to be their oyster, they both build their own respective lives, not knowing how each of the choices they makes affects the other, and creating the perfect storm for conflict once they realize they aren't alone.
When they do meet each other, things are initially cordial, with the two even becoming friends, sort of, and wordlessly agreeing to go their separate ways. Although on the surface they may seem identical, in the sense that they're both socially awkward bronies, things become more nuanced the further you look, and it's these small differences, along with an air of competition, that leads to each of them viewing the other with distaste, and a mutual desire to one-up the other.
After a love triangle is created and eventually explodes into violence, what follows is a clash of interests and belief unlike any seen before, and when Sunset and her friends find themselves fighting a mentally unstable, violent villain who seems to know everything about them, they're left questioning both their existence and the morals they've upheld their entire lives, and what it means to fight "for the greater good."
"In the service of sin" is a phrase that evokes various interpretations depending on the context. In this case, it refers to themes of moral struggle, temptation, or the consequences of engaging in sinful behavior. This phrase could be explored in literature, philosophy, or religious discussions, often highlighting the conflict between good and evil or the human condition.
(Story is written from first person point of view)