Every ruler needs a whipping boy/monster, someone who can be used to frighten your enemies but one who has to be executed after outliving his usefulness. In Game of Thrones, Lord Tywin Lannisters had the Mountain, a known rapist and murderer who he used to do his dirty work like killing some royal children and setting fire to the enemy countryside during war. The disadvantage of having monsters in your employ is that they would invariably become too ambitious or greedy and turn on you. The trick is to kill them after they are no longer useful and have a younger more obedient successor lined up. If you can use the act of killing them to gain some political capital so much the better. In Lord Tywin's case he sent the Mountain's head to an aggrieved party to make peace i.e the Dornish The disadvantage of doing this however is that potential servants would look at how you treated your last monster and be wary of working for you. A little deception might be called for in killing your monsters so that people will not think you're ungrateful and duplicitous.
