I'm looking at the half-fiend minotaur), which has Smite Good from the Half-Fiend template, but a Charisma modifier of -3.

Now, multiple people before me have asked whether a negative charisma modifier would apply to the attack roll when using Smite Good, and the answer is no. The rules text explicitly says that they add their "bonus (if any)" to the attack roll, and a bonus is conventionally understood to mean 0 or better. So a negative modifier is not a bonus, and therefore does not get added to the attack rolls.

But. Later in the description it says:

In addition, while smite evil is in effect, the paladin gains a deflection bonus equal to her Charisma modifier (if any) to her AC against attacks made by the target of the smite.

So using an exact, literal interpretation, the half-fiend minotaur could declare a Smite Good as a swift action, gaining:

• a 0 bonus to hit (no change)
• 6 to damage (= HD)
• a -3 deflection bonus to AC

Right? I mean, it's thematically appropriate for a bellowing minotaur to lose some defense in order to gain some offense against a particularly troublesome opponent. The imprecision of the language bugs me, though.