My theory is that the devil straight up lied to Sang-ah in order for him to get him to kill Woo-yeol fast so that nobody find out about his screw up.
And Woo-yeol’s “friend” knows this, but since he’s a doppelgänger who killed the original, he’s not going to say the truth.
These devils are amoral, but also If I remember correctly, the original is supposed to be someone who deserves to die, so the doppelgänger can take their spot.

The math isn’t mathing for me. No way Woo is a murderer, it’s probably going to be about technicality or somethin cuz his actions so far don’t match up with being a murderer