In the first episode, Subaru was asking about where was the beautiful girl who summoned him into this world... Except that it was a beautiful girl that summoned him, it's just that said girl is Satella, AKA the Witch of Envy.
Why does Subaru, of all people, have Return by Death despite being the weakest character? Because he dies often enough that he can come across the information needed to solve the events of the arc.
The anime ends with some unresolved plot threads, such as the royal succession and the witch's cult. However, look at the title of the final episode: "That's All This Story Is About". Those things are left unresolved because this story was never about the Myth Arc. The point of the story was to deconstruct the typical NEET/Gamer/Hikikomori/Otaku protagonist of these types of stories in the form of Subaru, showing what it would realistically take for someone like that to become a true hero in this scenario (the answer- A lot of hardship, work, and personal growth). By the end Subaru has grown as a person, becoming someone that can genuinely be called a hero and someone his love interest can realistically fall for. In other words, Subaru's character arc was "All This Story Is About".
All of the If routes represent Subaru taking up a certain sin.
Ayamatsu -> Pride as Subaru wants to prove he's better than Reinhard and does anything possible to achieve this.
Kasaneru -> Greed as Subaru becomes an Apostle of Greed and constantly dies to get a perfect ending regardless of the cost.
Rem If -> Sloth as Subaru gives up and takes the easy way out.
Oboreru -> Wrath as Subaru can only understand hatred, be it his own or someone else's.
Harem If -> Lust for obvious reasons. However, it is unknown if was as serious as the other Ifs as the author removed it.