I agree with the RP potential - the Lamenters for example, unwilling to kill brother Astartes and making honourable friends amongst the Marines Errant, loyalists who were then attacked by the Mantis Warriors in a deadly ambush.
The whole Badab War is a hot mess. There's a lot of internal conflict: Sworn to Oaths, but Sworn to Brothers.
@ Steel: You would..! (Akkad has a few people he didn't get on with, shouldn't be hard).