I think the reason why donghyun regretted so much is bc the night when him and taejun went in a bar and when taejun passed out he think of some indecent thoughts on him while taejun was thinking of killing his self already. When he learned the truth, he thought that maybe if he continued to be drowned by his desires (even though its bad) then maybe he can save taejun and will still be alive today but he didn't succumb himself to it that's why he end up regretting it. And I kinda understand why he said that yea there's too many what if's on his part (◞‸◟)

can someone like explain clearly to me what donghyun did to taejun and his lover that made him regret so much? i knew he took a picture but i dont have a clear picture of what happened after that @.@
and what are the consequences of his wrongdoings?