Ancient Rome's “Praetorian Guard” was an excellent example of what no emperor would ever want in his imperial guard- an elite and ruthless fighting force perfectly willing to overthrow emperors at will for its own ends. Of course, Rome eventually fell, but it was succeeded (in the eastern regions of what had previously been the Roman Empire) by the Byzantine Empire based in Constantinople.
The Byzantine Emperors needed bodyguards too, so they created their own elite guard force, the Varangian Guard. But the Varangians were different. Varangian Guardsmen were Vikings and Saxons, warriors with well-established traditions of extreme loyalty to their own chieftains. This loyalty was transferred to the Byzantine Emperor, and in all the years of their service to the Byzantines, there was only one instance of the Varangians interfering with the imperial succession.
This is especially remarkable when you consider that the Varangians stood to profit greatly whenever an emperor died. According to tradition, any Varangian guard was allowed to go to the treasury when the emperor died and run off with all the gold and treasure he could carry. Many of them even became wealthy men as a result, but they don't seem to have ever abused the privilege. They could have easily just killed the emperor themselves and set up a new one of their own choosing, which is something the Praetorian Guard used to do all the time. But they never did so, and the only misbehavior they were generally guilty of was drinking too much. The Varangians, it seems, were a cut above!