So I think that the Cabal are actually on a crusade [i]against[/i] the darkness, i.e. the vex/hive. Allow me to elaborate:
1. We've only seen the Cabal fighting the vex, which is described as "evil so dark it despises other evil".
2.They've only been encountered on Mars, a planet which we know the vex are attempting to transform into another machine world.
3. The Cabal have the ability to DESTROY WORLDS, if they wanted to decimate humanity, they'd have hit Earth a long time ago.
"But the Cabal attacked us while we occupied Mars". True, but we now know that the vex were there long before we were, and maybe the Cabal knew this, and in an attempt to destroy the vex were forced to attack us to get us to flee Mars, as to escape destruction.
"The Cabal destroy planets and moons just for getting in their way". Were they just 'in their way', or were they darkness/vex inhabited worlds?
And now we have evidence of the Cabal meeting Oryx head on, crashing their very own ship into his dreadnaught on a suicide mission to stop or slow him.
I believe the Cabal aren't necessarily bad, and that they're really on a mission to stop the darkness. I don't think they ever meant to "step into a war" with us guardians, we were just too zealous to allow them taking Mars from us.
Also, in every encounter with the Cabal (disregarding the assault on Mars), we were the aggressors.
TLDR:
Cabal = good guys with a plan
Guardians = misguided superheroes
Edit: So, after a month of this post being made, and the hundreds of comments it's received, I've concluded that the Cabal are indeed, bad. Yes they were running from something, but then collided with us and couldn't conquer us, so they tried to steal the Hive's tactics since they have an effective strategy to combat our ghosts.
Now with Oryx, AKA: the darkness, off their tail and an emergency transmission sent directly to the Cabal Empire, I think we'll be seeing a rather large response team in the future.
Cabal are warlords. They just want to conquer. They tried to conquer Mars, but began a war with the Vex there to gain control. We happened to come along, and seeing us trying to reclaim Mars, they began to fight us too.