I want to say that I don't really think that LS wants Sith destroyed, and DS wants Jedi destroyed. During my quest line, my actions of letting Jedi flee instead of killing them was considered a LS act (I am playing Imperial Agent). LS/DC determines your characters alignment, it's not connected to Force in any way. All classes have DS/LS points. A Sith Warrior can be fully 'light' aligned.
Although, while I was playing Jedi Consular during the beta testing, leaving a Jedi couple in love without letting the other Jedi know about it turned out to be a DS act (since learning the ways of the Force doesn't allow such relationships, at least that's what those 'Jedi masters' told me, which I find wrong). For such classes, or just in certain situations DS/LS is connected to the Force, but all in all, it's not, and shouldn't be.