I'm currently about 1/5th of the way through The Horus Heresy and have read that book. The Dark Angels were just one of the founding 20 chapters of genetically enhanced Space Marines. They remained loyal to the Emperor and the Imperium rather than falling to the evil of the Chaos Gods from the Warp.
From her own mouth she said she got brain damage from one of her drug overdoses. She's a couple sandwiches short of a picnic these days.