>>96053418
It was in Sons of Fenris, the fallen Dark Angel was Cadmus. He organized the local PDF though and was generally well liked, but was working with the thousand sons as a traitor. He basically calls the marines retards for engaging in the Eternal War because it was pointless cyclical endeavor that changed nothing and he thought he accomplished more as a local guy doing good than any marine fighting other marines in pointless battles ever had and allowed himself to be killed without fighting.