Okay so Meros, apothecary of the 9th company under Captain Furio of the Blood Angels during the Horus Heresy. Probably THE MOST BAD-ASS of all Blood Angels , he not only saved his own primarch from eternal damnation to the forces of chaos, but also met his gene brother from TEN THOUSAND YEARS IN THE FUTURE (Brother Rafen). Alright so this hero of heroes was just a line-astartes and a junior medicae and was tasked by First Captain Raldoron with the removal of the progenoid glands of Brother Alatros who had succumbed to the red thirst , which at that time was a secret held by Sanguinius and only a few other blood angels (and Horus, the sneaky bastard). He was told never to talk about what he saw and witnessed Raldoron painting Alatros' armour black, he spoke the words "You are in the company of death. I hope you find peace there." Foreshadowing of the death company? I think the answer is obvious.
Later, during a war with Dark Eldar, the thunderhawk that Meros was in was shot down and he slipped into a coma. Two of his squad mates placed him into one of the exsanguinators, the blood angels coffins used for transforming neophytes. The intent was to drain out the poison from his blood and replace it all with fresh blood. During this weird, blood induced future-sight hippy trip, Meros met Brother Rafen, a current sergeant of the blood angels chapter who has the same gene-seed as Meros. suddenly they realized that the Sanguinor was there with them and a whole bunch of crazy shit happens and Meros and Rafen have to fight a horde of phantoms and vanquish them with little effort. Realizing the magnitude of meeting his gene-ancestor, before being pulled from their separate but parallel blood comas, Rafen uttered a single warning, a curse: Horus. When Meros awoke, it was to find his Legion being called to the Signus System by Warmaster Horus. And just like that, Meros's butthole tightened into untrust for Horus and knew that bad fucker was up to something to no good.
Now after all the crazy shit goes down on Signus Prime and Sanguinius and a small strike force have broken into the Cathedral of the Mark, The greater daemon of Slaanesh , Kyriss , had helped to create a cursed artifact of pure rage . The daemon made a deal with Sanguinius that all he had to do was give himself over fully to the rage and corruption of the artifact and Kyriss would cure the Blood Angels of the black rage and red thirst forever. Seeing that his primarch was actually considering accepting, Meros leapt into the artifact instead, followed by his loving father Space Jesus. Inside this realm of Chaos that was the artifact, Meros refused to let his primarch take the deal, so he CUT OUT HIS OWN GENESEED and gave it to Sanguinius before letting the rage overtake him. He sacrificed himself for the most beloved Primarch in the Imperium, be thankful!
Meros the badass apothecary was dead, and The Red Angel was born. Not that pussy Angron. The Red Angel was a being of pure rage (Seriously, the motherfucker was literally on fire), he even told everyone if Angron had a problem with him taking Angron's title, then Angron could come try and take it from him! (Angron never gave a fuck, considering he hated said title.)
(Had Sanguinius been corrupted both him and the Blood Angels would have been lost. Horus would have won the Heresy!)