I actually think The Speaker is a power hungry cult leader trying to use the travelers power for his own goals. I think the Stranger is part of a resistance group of sorts, trying to bring about a balance of light and darkness.
What I do know is that you don't really get much information about anything that is going on. You are brought back to life, yet you don't know why you were dead in the first place.
Now I do find it strange that when you headshot The fallen they have what appears to be light escaping, the vex appear to use light infused crystals, but the cabal and hive have a wave of darkness that escapes.
You also don't receive much help from the Warmind Rasputin... and that has me thinking, what if Rasputin is in league with the Vex.
I do think the Speaker is just using you, and will later turn on you.
So many possibilities lol