The philosophy of Echidna's greed vs Subaru's stubbornness
### Regarding the contrast between Echidna and Subaru in Arc 4 Echidna's proposed contract was logically the most efficient solution with the minimal statistical casualties. However, it was predicated on completely grinding down Subaru's soul. Subaru refusing her hand was solely because he chose 'foolish stubbornness that refuses to accept anyone's sacrifice' over cold efficiency.