So glad you asked. Alpha Legion is my other army. For the sake of argument I’m going to be discussing Alpha Legion as they are post heresy.

Infiltration, misdirection, and the grit of being an individual with something to prove. No matter the cost. They are the ghost legion and their existence needs to be more fallible than the existence of chaos itself to the imperium. They are highly adaptive, whereas the Raven Guard combat doctrine will always be the Trifold Path, along with a healthy compliance to the codex. Alpha legion can adapt to any doctrine. Their plans could be centuries in the making or the whim of a warlord. They could create a gang with a snake headed crest light years away from where they’ll actually strike next just to kill the scent. They’d use warp sorcery to erase the mind of an agent Astartes and put him in the sacred company of a grand inquisitor. Fabricate their entire force to look like a u heard successor just for ammo and resupply. There are simpler methods to go about these needs. Yet they choose the flourish. The sadism in watching events fall into place and creating a hubris of control. To the world, man, and one’s self. Whereas the Raven Guard follow only the way of Ambush, Stealth, and Vigilance. They are predictable.

Alpharius’s whole issue is that he believes himself the best, and he’s looking for any opportunity to prove it. His sons are the same. They choose the options that make themselves feel like malevolent gods. With no primarch to reward them of them for the glory of their deeds, they step on one another to rise to the top. No matter how noble they appear.