A creature who, because of ability damage from a disease, reaches [Constitution 0][1] 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][2].

**Corner Case**  
[Constitution](http://www.d20srd.org/srd/theBasics.htm#constitutionCon) (*PH* 9) says the ability modifier is added to "[e]ach *roll* of a Hit Die (though a penalty can never drop a result below 1--that is, a character always *gains* at least 1 hit point each time he or she advances in level), but later says, "If a character’s Constitution score *changes* enough to alter his or her Constitution modifier, the character’s hit points also increase or decrease accordingly."

Emphasis mine.

Thus, oddly--and, perhaps, luckily for those peasant villages beset by ghouls--, a creature with *only* a maximum of 1 to 4 hp can be at negative hp and subsequently die (were he not stabilized) despite still having a positive Constitution after contracting ghoul fever. Were that the case, I'd argue hp loss is still the killer (critical existence failure on the creature's part, perhaps?), not the disease proper.

  [1]: http://www.d20srd.org/srd/specialAbilities.htm#abilityScoreLoss
  [2]: http://www.d20srd.org/srd/monsters/ghoul.htm