This might not be %100 but this is how I understood it: Roy almost killed Jinwoo in a literal sense. He attacked him (trying to protect his sister) and gave him the wound on his abdomen-- now covered by the tattoo. While injured, Jinwoo fell of the cliff/mountain and caught himself by flying for the first time. That's Roy "saving" him- if he had never attacked him Jinwoo never would have flown. Jinwoo's "revenge" was him shooting Roy with the arrow (an eye for an eye type situation). He then repayed Roy for "saving" him by taking him in and not killing him.
I see. That does makes sense
I'm a bit confused of the AU. Jinwoo says Roy almost killed him and saved him. I get the saving part since it appears that encountering Roy somehow gave Jinwoo courage to fly(?).... What about almost killed and getting revenge part