Just to clear some things up,,
Taeyong killed Jaehyun because he was jealous, yes he was selfish and all but he couldn't stand the way how everyone always flirted with him and the way he flirted back, he couldn't stand how no one paid any attention to him.
Even his own family would care for Jaehyun more then him and it was as if Taeyong never existed.
But Jaehyun isn't so innocent himself. He was texting another girl behind Taeyong's back before he and Taeyong even got together. Taeyong found out and was heartbroken but he stayed silent and never confronted Jaehyun about it.
So Taeyong thought that if he can't have Jaehyun then no one can can have him and so without any quick thoughts once he saw the opportunity that the river gave him, he pushed Jaehyun in, knowing that he didn't know how to swim properly.
He never meant on doing it but he just couldn't stand it anymore.
And for the dead Taeyong part, well Taeyong is actually a returned soul. He killed himself but his soul was rejected to the gates of both heaven and hell since he never did anything good for humanity nor did he do anything terrible so he was sent back to live another life again.
One thing about Taeyong is that he isn't really fazed by anything and the thought of death doesn't really surprise him anymore since he already "killed himself" once.
Jaehyun's soul however was accepted since he was killed by a loved one that he cared for deeply but his sins of talking to another girl behind Taeyong's back got him into Hell instead. And so Satan, his boss allowed Jaehyun to get revenge on Taeyong since Jaehyun didn't see anything coming so he had a right to get revenge on Taeyong cause he could have just broke up with him instead of killing him out of all things.
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If you're still confused then feel free to ask me anything here!
