Assassin missions?

[VirgilCaine]

Oh, never forget the movies Grosse Point Blank and Assassin (the one where the assassin starts taking care of the kid and training her).

Uh, Leon? The Professional?

 

[Grunk]

My campaign features an evil Assassin PC. Oh what fun I've had DMing those encounters. As posted above, the challenging part is usually getting away undetected or making it look like an accident.
I use world history as a big inspiration for ideas. This is very useful for your situation. For example, the assassin and the party were hired to assassinate a young dwarf nobleman. This caused a HUGE war between the dwarves and the local human nation. The inspiration of course is the assasination of archduke Ferdinand and the begining of WWI. History is a great source!