Board Thread:Warhammer 40k General Discussion/@comment-72.93.49.215-20140912200020/@comment-18016240-20140915155144

Sanguinius' death and Ka'banda's (I think that was the bloodthirster's name) curse made it worse, but there were already isolated incidents concerning battle brothers within the Blood Angels legion going mad from the Red Thirst and Black rage. (There's articles on both the red thirst and black rage within the wiki itself if you want to check.)

Those afflicted with it during Sanguinius' life and leadership were executed and their gene seed wasn't used, which limited it's ability to spread, but the events of the Battle of Terra and the losses the legion suffered during the Horus Heresy itself caused them to start using the afflicted battle brothers' gene seed to help replenish their losses. This set off the continual mutation and degradation of the Blood Angels' collective psyche to the problems we see today.