CDG is a tool.
How you use the tool matters.
I generally run through a three tiered criteria before an NPC uses CDG.
1) Do they have at least a 5 Intelligence?
2) Do they have time to conduct the CDG?
3) Do they have a motive to CDG?
Now the third one is usually the foggiest one to puzzle out, because motivation could be anything.
Their opponent is a cutthroat, he's gonna do what he's gonna do. Maybe it's an arch-enemy of the party bent on their destruction. Maybe it's someone who they foiled them in the past. Maybe the individual is desperate for money and doesn't want to leave a witness?
I also an unofficial fourth criteria, which is usually that the NPC be non-good. I tend to rule that CDG is a decidedly evil act, and most good NPCs wouldn't do it even to a foe. If the foe dies from standard combat that's one thing, but slitting their throat while they're helpless seems like a bit much.
Why the int cap?Animals do this all the time if they can manage it.