My theory (●'◡'●)

Ok so my theory is that the brother is the one that convinced Jeong-hyeon to kill Tae-hwa,in order to prove his love. This all happened in the past of course. My reasoning is because of the letter that Jeong-hyeon found;" you who loved with a black-tinged heart, must prove your love by staining with his blood. For when your love has been proved true, eternity will come to an end." I don't know how to connected the first part but the " you must prove your love by staining with his blood" is what I imagine what the brother said to Jeong-hyeon for him to kill Tae-hwa. I imagine the brother said this to Jeong -hyeon in the snake form, the same snake that appears when tae-hwa was becoming an immortal. Although my other theory is for the last part " for when your love has been proved true, eternity will come to an end" this makes me think that the message was for Jeong-hyeon possibly because he was the one that was an immortal back then and wanted to stop being an immortal so he killed Tae-hwa. And the reason as to why he had to kill Tae-hwa is because it is the only way to stop being an immortal, since present Tae-hwa also wants to kill Jeong-hyeon in order to stop being an immortal. Or am I just overthinking this lol, idk when I come across good series like these I think of many possibilities


Also what do yall think it meant by " you who love with a black-tinged heart" like I'm very confused as to what it means. ٩(๑❛ᴗ❛๑)۶