Jake didn't want Gren dead, but he's always associated him with danger because of his connection w Ross & how unpredictable he was - it's a given he'll associate him with trouble for the man he loves.
Also, Jake is the one who helped Gren escape death once, offering him the chance for a new life ; but Ross ended up finding him and that was that. In the end, Gren got a proper burial, thanks to Jake once again. :/
=> About them fucking right after.. yeah I get your feeling, it was sudden for everyone else LOL. But it was more of a coping mechanism to stop thinking about it. I get why Shin didn't want to sob in front of Jake, and Jake went along with it.
i keep skipping chapters in this manhwa because the shit they do is js so fucked up i cant imagine fucking ur fuck buddy (who wants ur bro dead) right after ur bro dies also, the amount of times they fuck to make-up is uncountable bro im gonna cry, every characters in this manhwa fine asl tho