Xenophilia. Let's not be dicks.
I almost including a (less profane) version of the "cabal of wizards" line when describing why the Cursed Founding was arguably not such a great idea 
Also, yes, +1 Xenophilia.
Haha I don’t think this one is a difficult choice.
+1 Xenophilia
Guilty of
Xenophilia, your Chapter brought shame to the
Silver Skulls and was declared Exocommunicate Traitoris. The circumstances of their betrayal of mankind remain shrouded in mystery to outsiders and much remains to be determined. Were they pursuing a prophecy of the Prognosticators, a victim of the Cursed Founding forced into a situation where they had no choice, or was it as simple as a mere moment of mercy breaking decades of hypno-indoctrination?
This question and more wait for answers, but if you'd prefer, we can continue setting the foundation for them.
How mutated is their gene-seed? Because almost anyone would prefer less mutated gene-seed and vote accordingly, this'll be a straight, take-it-as-it-falls roll...
6
The renegades have (at least initially)
Pure Gene-Seed, remarkably unscathed even in their treachery.
Given their gene-seed is mercifully free of mutations and flaws, we can move on to the Astartes themselves.
Who is their Figure of Legend? He is foundational to the Chapter's mythos, the example each member of the Chapter looks back on and aspires to be more like.
11, 98, 9, 10
A
Battle Brother of
The Silver Skulls, 9th Company, 10th Squad. How they came to be revered by the Chapter is surely a legend in and of itself.
43
The Chapter Master, trusted to lead the Chapter through thick and thin. He must've been a fearsome combatant and venerable leader, like most in his position.