In fact, while there are multiple endings planned and the body counts may vary, it looks like literally any person, droid, or alien who can talk will need to be silenced as you terrorize worlds such as the lava-spewing junk planet Raxus Prime, forested Wookie homeworld Kashyyyk, or the glowy, mushroom-y swamp of Felucia. Note that each planet is jammed with new stuff for you to wreck.
Above: We've been warned that the Jedi riding rancors thing might not happen - but we hope desperately that it does
Specifically, aside from Vader, we know of only two characters - in the whole galaxy - whom we can confirm you definitely don't have to kill. One of the chosen two untouchables is Juno Eclipse, the sexy blonde pilot of your ship, the Rogue Shadow - she's also your love interest, incidentally. And you're doing well - check out her art in our screenshot viewer. The other is a bipedal droid whose name is still secret. We were intriguingly promised "he" will supply the trademark, Star Wars droid-style comic relief and some of the most "empowering" moments in the game.
Now, remember: Force Unleashed gives you the ability to levitate and spontaneously combust an AT-ST, or (maybe) to kill a rancor by getting swallowed and then exploding its entire midsection from the inside out with a gooey Force Repulse. With that in mind, the fact that one particular character is described as empowering means something big, indeed.