Criminals and unrepentant terrorists tend to get a bullet in the head.
I'd also add "people who betrayed/played me, personally" to that list, but pretty much the same for me. Light choices in most encounters, only going Dark when I decide that someone just needs to die.
The problem is that the writers weren't very consistent. In most encounters the light side option will be of the "capture, not kill" variety, which is a philosophy that makes perfect sense for an Agent; don't throw away a good tool simply out of spite, or for the fun of the kill; that's what the Sith do. So if your Agent believes in making the Empire a stronger power, the Light option is often superior, and it's fun to rub that in the faces of the Sith.
But then you have a few where the Light option is to let someone who's sworn to destroy the Empire go free, rather than kill them. It's nearly the exact opposite of the previous situation; only a complete idiot would let their enemies walk away just out of a sense of niceness when it can only serve to make the Republic stronger. So going pure-Light or pure-Dark just doesn't make a whole lot of sense, given this disparity.