Sneaky NPCs, the Malnosso are. You never know what they'll want from us.

One argument to be made then is that if one vaccinates everyone else (eventually, granted), that his death is also unnecessary. If we end up with a cure, the enclosure does not explode - hopefully - and the resources to produce enough of it for everyone should no longer be an issue, with the attack-and-disease combo threat over.

Maybe we don't know this yet: is the survivor someone who lives in Vaskoth?

There's also the implication that with the cure, they will (potentially) just be producing a number of other Patient Zeros: immune but carriers.