HERETIC!!! I will drink your blood for such slander!!....er...uhm, I mean we are ever the Emperor's servants! All kidding aside this link should help illuminate you to the glorious truth of the Sons of Sanguinus: http://warhammer40k.wikia.com/wiki/Blood_Angels
To answer your specific question the blood angels first "screamed for blood" on sigmus prime during the heresy. Horus (his perfidy still unknown to Sanguinus) sent the Blood Angels there to "deal with a rebellion". By the time they arrived in system the world was a daemon infested charnel house. Sanguinus got messed up something fierce by a bloodthirster. Instead of finishing off the angel he instead killed hundreds of his marines right in front of him. The psychic backlash made sanguinus black out. With that many battle brothers slain in front of them and their primarch laid low the rest of the BA...Freaked the F**k out! When sanguinus came to all the demons were dead the Bloodthirster driven away and the chapter shaken by what they had done.
The issue was compounded by the loss of Blood Angel geneseed imprint during the Mars/Terra campaign of the Heresy, forcing them to recoup it from their, now dead, primarch. Either his death (the more accepted scenario), or the events on Sigmus, possible both imprinted the genetic memory of those events into Sanguinus' DNA, cursing all future generation Blood Angels.
They have always been an assault oriented army, before the heresy, rivalry between the Blood Angels and the World Eaters was fierce in this regard. They deviate slightly from the codex company scheme in a couple ways. The 1st company (veterans) are always deployed as assault when not in terminator armor. The 8th company is a reserve assault force. When not deployed as assault squads they are normally the pilots of the fast moving vehicles of the BA (Baal preds, assault bikes, landspeeders, and rhino crews).
Now then, on to your warp ridden lies and slander!
The Blood Angels, White Scars, and Imperial Fists were the only chapters to stand with the Custodes and IG throughout the entire siege of Terra. Sanguinus himself held the Ultimate Gate alone, striking down the Bloodthirster who smacked him around on sigmus in the process, while the rest of the imperial forces moved the wounded into the palace. He then, knowing it would cost him his life(fortelling is a bitch in that way), teleported to face horus at the Emperor's side. That, my misguided friend, is loyalty.