##Assassination can be regulated
In the venerable long-running Foreigner book series by CJ Cherryh, the central action takes place on a planet where the dominant non-human civilization has institutionalized assassination, to the point where the well-regulated assassins' guild is considered a respectable branch of both local and centralized government.
Note that this book series is not comedy, but serious "anthropological" science fiction.
From the Wikipedia article linked above:
[...] assassination is a legal and accepted means of settling disputes, provided proper protocol is followed. One files a document of Intent which liberates the target to file one back. The Assassin's Guild has the right, often exercised when Intent is filed for foolish reasons, to reject a particular filing.
##Assassination can be ethical
When compared to murder (unjustified killing without following the proper assassination protocols) and warfare (which involves mass killing and potential collateral damage and destruction of property) it's easy to see how a society might develop a tolerance for assassination, and even a respect for it as an institution.
A common theme in the Foreigner series is that assassination is considered (by the non-humans) more elegant and moral than resolving disputes through warfare, since innocent bystanders are less likely to be injured or killed.