Well if you think about it you can't be dark since this game is based off the light side ending. And there are mods out there to swap around the npcs to play as a bad guy. Just dl the reborn mod and it does it for you.
But if you kill Lando or Jan then mission is failed and how that suposed to work explain
It was a joke that you can choose to kill them and be bad, but the game ends because you need them.