This is interesting to me as well, my group decided that (for a stopgap game), they want to run a group of assassins.
There are the obvious ideas (kings, nobles, merchants, and other people of importance; criminals, murderers, assassins, mercenaries, and other criminals against the state or revenge killings; evil cult leaders and wizards deep inside "classic" DnD settings; People in posession of things of major value that can't be easily stolen; People who know something that they "shouldn't know").
For less obvious ideas I've come up with:
Someone who orders an assassination on his own head, and offers up a substantial reward for whoever pulls it off. A real high-profile job perhaps, or maybe just a set-up to get revenge on the assassin guild? Perhaps he's already dead? All three you say?
Two rival merchants that try a bidding war in order to have the other killed.
Assassinating a particular individual, with the requirement to return the body. Of course, the person offering the job is a necromancer, with some very dangerous plans for this particular corpse.
The "Good" aligned high cleric and the "Good" aligned arch-druid each want the other dead. For a not-completely-evil party that could raise some questions.