The Blood Angels gene-line is affected by a dreadful flaw, which makes its brethren susceptible to the Black Rage and the Red Thirst.
Via our good friends at Lexicanum
The gene-seed of the Blood Angels (and its successors) is still capable of producing all nineteen organ implants that facilitate the transformation of a normal man into a Space Marine. It also has the unique quirk of vastly extending the lifespan of the Space Marines of the gene-line, and it is not unknown for a Blood Angel to live more than a millennium. The gene-line, however, is also affected by a dreadful flaw, which makes its brethren susceptible to the Black Rage and the Red Thirst.
All of the Chapters derived from the Blood Angels still suffer from the psychic imprint left by Sanguinius’s death. This can cause them to go insane prior to or during a battle, and they believe that they are Sanguinius himself during the Battle of Terra. The condition is irrevocable (except for one case, Chaplain Lemartes, who is largely able to control it), and victims are locked away in the “Tower of the Lost” on the Blood Angels’ homeworld until they finally die. This insanity is known as the Black Rage. When a Space Marine is overcome by the Black Rage he is reborn in a world of anger, hatred, fury and nothing else.
As well as Sanguinius’s memories, the Blood Angel is touched with a small portion of his unearthly power, boosting his strength and vitality to superhuman levels. Rather than face a slow, insane death, Blood Angels will form those who have newly succumbed to the Black Rage into a special unit known as the Death Company. They paint their armor black, daubed with red crosses, signifying the wounds of Sanguinius, and are usually led by the few Chapter officers, notably Death Company Chaplains, who are able to communicate orders to these lost warriors. They are then sent out to perform the most dangerous assaults, hoping for a quick and honorable death in combat. The other name of Black Rage – the Flaw of Sanguinius.
The Red Thirst is one of the two genetic flaws in the gene-seed of the Blood Angels and their successor Chapters. This causes warriors to suffer an unusual bloodlust, instilling them with a thirst for the blood of their enemies. Those who succumb to the Red Thirst or Black Rage are either formed into the Death Company or locked away in the Tower of the Lost on Baal, until they change completely, going insane. These traits have also been passed on to their successor chapters, such as the Flesh Tearers, Blood Drinkers, and Angels Sanguine. One successor chapter, the Lamenters, was until recently thought to have managed by unknown means to eliminate the flaws from Sanguinius’ gene-seed, but has been stricken by extraordinary bad luck in battle and as such has been nearly annihilated.
The Red Thirst is so overwhelming that the Blood Angels forget their noble heritage and completely lose control over themselves. It is not unheard of that Blood Angels squads abandon advantageous positions to engage the enemy in hand-to-hand combat. More often than not, these assaults turn out to be so devastating that unprepared enemy forces are simply swept away. The Red Thirst was a genetic flaw deeply embedded into the Blood Angels Gene-Seed. When Sanguinius became aware of it, he felt responsible for the suffering of his sons and desperately sought out a cure while keeping it secret from the rest of the Imperium, including the Emperor himself. While isolated instances of Battle-Brothers succumbing to the Red Thirst occurred throughout the Great Crusade, it was not until the Battle of Signus Prime that the entire Legion suffered from it.
