It really depends on the era and the type of nobility. Most of the European royal families were related in the 18th and 19th centuries--I can't think of too man

It really depends on the era and the type of nobility. Most of the European royal families were related in the 18th and 19th centuries--I can't think of too many instances of them killing each other with the gusto of, say, the leaders of the French Revolution. Exceptions like the killing of the Duke of Enghien blackened the reputations of the perpetrators, in this case Napoleon and Talleyrand. The more interesting counter-examples are cases like the succession struggles in the Ottoman Empire, which after a couple of generations ran on the Highlander principle. The last brother standing became the Sultan.