perhaps, but perhaps...
if both of them live, at least now both of them can work together a new method to completely destroy the curse
I think now they can ask the MC's mom to help them since it has been told before that MC's mom have some kind of spiritual power and there's also shiren's parent who already have some experience in cursed stuff
in my opinion, working together with Shiren who can identify the curse may prove to be a better choice in the long run to completely stop the curse
however... the loli died, oof
Edit: i just realized they say the curse will disappear in some time, tough before it completely gone, it's going to keep killing people
so we can say its a matter of completely saving some cursed people by letting Shiren die, tough after Shiren die nobody can give the rest of the cursed people some extension time again,
or give some extended time to every cursed people (which all of them soon die anyway) by letting Shiren live