Blackpetal, the answer is quite simple. You kill someone out of punishment or vengeance, but also to ensure that can no longer betray you again.
On the contrary, you maim someone our of punishment or vengeance, but more importantly so that they can drift about and serve as an example for those who may be engaged in or contemplating taking the same types of actions.
Naturally, this is entirely theoretical and has aboslutely no bearing on the matter at hand.