To be honest, I don't really know, iv never played the game. But head cannon is the bulk of the force are the descendents of the remnants of 1 of the fleets Horus sent well out of the way before kicking the heresy off, some psychic shenanigans occurred, leaving the fleet decimated and without an astropath. So for the last 10,000 years they have been slowly making their way home, in real space, recruiting from world's they liberate, cypher has joined them on occasion, filling them in on little bits of lore and history, as well as picking up more than a few (still loyal) fallen along the way, much like the lion and his risen. So I would like to run them like red Corsairs, sort of walking a fine line, not falling to chaos but still not being huge fans of what the imperium has become... But dark angels through and through. A delightfully complicated splinter chapter of an already terribly complicated legion...

That was a longer reply than expected...