I think this song is based on V s character in the HYYH ( BTS universe) where Taehyung’s father is abusive towards him and his sister after their mom died. one day he comes home to find his dad abusing his sister and ends up killing him trying to save her. The song talks about the burden he felt all the time he suffered from his father ,the feelings that he hid from everyone while he was being abused (maybe he tried to “play it cool” and not show his friends and sister the hardship he is in) and he feels sorry towards his sister,father and friends for different reasons. He now sees himself as a sinner and blames himself for his sister’s pain(because he was not strong enough to protect her only because he couldn’t stand up to his father ) and therefore he wants to erase his sins but he cant do that and now he just asks for forgiveness.

However, some people have stated that it could be something relating to sexuality. (Not in storyline, rather a personal story, considering he wrote the song lyrics himself.) But that is just a theory, because V of BTS has not spoken about it since the release of the song.