Up to the 1300's, the Norwegian kings had a dedicated secret police/death squad as part of his court. The mission statement of the "Guests", where "To search the realm for enemies of the king, and kill them.

There are certain other housecarles at the king's court, who, in addition to the housecarle's title, have a by-name and are called "gests." They have this name from their manifold duties; for they visit the homes of many, though not always with friendly intent. These men are also in the king's pay and get half the wages of "hirdmen." These are the duties that belong to the office of these men: they serve as spies throughout the king's domain to make sure whether he has any enemies in his kingdom; and if such are found, the gests are to slay them, if they are able to do so. But if the king sends his gests upon his enemies and those against whom they are sent are slain, they are to have for their trouble as much of the enemies' wealth as they can carry away at the time, only no gold, for that is the king's, as is all the rest that the gests are unable to bring away. And whenever the king becomes aware of an enemy, it is the gest's duty to pursue the foeman and thus to cleanse the realm.
-the kingsmirror, 1260


http://www.myarmoury.com/talk/viewtopic.php?t=11767

Another item from that thread:

In one of the Viking Sagas, a fellow bursts into a house with a spear, and settles a dispute by ramming the spear through his sleeping enemy, before vanishing back out into the storm.

It wasn't only one of the Viking Sagas. Heck it happened numerous times in Burnt Njal's Saga alone! Hallgerd, Gunnar's wife, sends a man to assassinate one of Njal's men, and Njal and Gunnar meet at the Thing and Gunnar pays the geld. Then Bergthora, Njal's wife, sends a man to assassinate Hallgerd's assassin. Njal and Gunnar meet at the Thing and Njal pays the Geld to Gunnar. Then Hallgerd sends another assassin to assassinate Bergthora's assassin, and you guessed it, Njal and Gunnar meet at the Thing, and Gunnar pays the geld. This goes on for five rounds!

Later on in the saga, men are assassinated simply by someone walking into a house, drawing a sword and killing the man. It seems as though this was more or less commonplace.

I found a Penguin Classics copy of Egil's Saga in our voluminous library, and there's a passage in chapter 18: "Thorgils returns from England":

Sigtrygg and Hallvard were in charge of all the King's missions, both in Norway and abroad, and had undertaken many a dangerous journey either to execute people or confiscate the property of those the King had decided to attack. The brothers had a large band of men and weren't very popular with most people, but the King had high regard for them.

They were Gest, clearly!