I'd like to answer as fully as I can, but I'm not sure I understand the question fully. Let me say this: I could add an option "hey, Lynn, I killed all those losers already, they accidentally were in my way, and, you know, Lloth demanded I killed them all, yay!" - but it might seem a bit illogical(after all, most targets are fairly smart/dangerous people who are in hiding, so you have to find them before you kill them), which is why I'm reluctant to add such an option right away.
(If you really just want to do the last mission, or to be able to kill all the guys at once, and other people would like it, too, I'll think on it).
I understand that you're saying that Assassinations may not offer enough "Murder, death, kill! Murder, death, kill! Murder, death, kill, MYAHAHA!" options. Like, real Eeeevil, not just "evil". I think you've just offered a brilliant idea - some other quest giver that would be encouraging you to wipe out entire Lathander temple, or entire Radiant heart compound, or butcher all the commoners in the Docks. That would be fun to play(though maybe at least once).
Unfortunately, there are some technical limitations. It means that once you start attacking civilians/certain groups, most people around you will go hostile, and you might miss on many quests, which the long-fans of the game may not want to do. And it's very possible to screw your main questline, too. (And even if not, I remember my "wipe everything!!!111" runs - it ended with me standing in front of the empty Promenade battling groups of guards and feeling rather... empty).
Maybe there's an alternative: a quest giver offers you some opportunities for massacre in new quest areas, where you can't screw the main game. This is something to think about, certainly.
(And in case you haven't tried it already, I really recommend you to download SWTOR(it's free!) and try to play for the Empire side. Especially for a Sith. You won't be disappointed!)