Responses

in asian mafia culture, the pinky is usually cut off after one has failed/betrayed at something/someone. in this case, moonhyuk cutting of his finger means he basically gave his life (as a missing pinky will basically cut you off from society, since you are being seen as a gangster) for chagang even tho he was already dead and devoted himself fully to him - moonhyuk didn’t fail or betray anyone (though he probably feels like he couldn’t save his Hyung) but was ordered to cut it off probably just for shits and giggles ...
Y the pinky tho, y does seojin want it, y does the sus dude have it, y is seojin such a baddie, we'll never know (人 •ᴗ•)