For example, in Throne of Bhaal, you can get a former enemy to join your party. They're still evil, but not insane anymore. For various reasons, you have the power to extract an oath of loyalty from them that is magically binding.
But you also have the option not to, and both options are equally valid (in that you can redeem this former enemy based on your actions, with or without the oath). And the interesting thing is, their reasons for remaining loyal even if you don't get an oath out of them or convert them to "good" are perfectly sound, too. And there's multiple reasons, not one.
A rare example of there being four possible ways you can interact with an enemy turned ally and there will be perfectly good reasons in each for why they stay an ally. (That being said, it was revealed by the developers that they ran out of time and they'd intended that if you treated them like crap and didn't get an oath out of them, they would not only stay evil but turn on you.)
And then there's all the "little" betrayals you can get in such games, where party members remain more loyal to their principles than to you, not bringing you to harm but acting against your wishes (such as a merciless law enforcement officer killing a criminal you've forgiven and refused to finish off after defeating them).