jay realised that taemin feels immense guilt for the death of his mother and his brother especially that makes him suicidal and want to give in to dying if the chance presents like it happened in the previous chapters where when he was stabbed he could have received jay's call on time and could be saved but he let it go and decided that instead of fighting to live he would give in, and that made jay very angry and kinda psycho, hence in here he mentions that now he's gonna make taemin feel guilty for him as well so that he can't give up on life so easily and live for him. hence all that drama, because jay can't figure out any other way to not taemin give up on his life when it's this easy to lose him
why did he do that? kinda forgot what’s been happening