A creature who, because of ability damage from a disease, reaches Constitution 0 is killed by the disease. If a creature has -10 hp before that happens, the creature has, instead, died from having -10 hp (usually via injury).
In your example, the wounds from battle killed him, not the ghoul fever.
Corner Case
A creature with only 1 to 4 hp could end up at negative hp and subsequently die despite still having a positive Constitution. Even then, I'd argue, hp loss is still the killer (critical existence failure on the creature's part, perhaps?), not the disease proper.