World Eaters
OK, the World Eaters are the second angriest group of marines ever, second only to the Angry Marines. Their Primarch's name is a pun on the fact that their so damn angry (Angron).
Before they were reunited with their Primarch, Angron, they were known as the War Hounds. When they were reuinited with their Primarch, they changed their named to the World Eaters, the 9th Legion reunited. Angron added implants and neurosurgery *Partial Lobotimisation) to remove the ability to feel or care about fear, but increased their Aggression to large amounts. The Emperor banned this practice when the World Eaters exterminated all life on a planet in one night. Horus, seeking to turn the Primarchs to his cause, didn't have to do much to get Angron to side with him, as all it took was to tell him that the Emperor was weak and to apply to his martial pride. When the Legions of Horus attacked the Imperial Palace, the World Eaters were at the forefront of the Chaos Marines, seeping into the breach and killing the most inside the palace.
The World Eaters are all Berserkers according to the flop of a codex that is Chaos Space Marines 4.0 , once again going back to the partial lobotomy, that only they seem to have access too. They use Chain Axes, which are just close combat weapons to 4.0 now, and used to be some of the most brutal hand to hand units in the game, but lost their feel no pain, and have to be mechanized or they won't be effective.
A hell of a guy by the name of Kharn comes from this Legion, hell of a guy that Kharn is...
The World Eaters are not organized anymore, as Kharn seperated the Legion on Scalathrax when the World Eaters were kicking the Emperors Children Legion's asses. Kharn got pissed off at his fellow Legionaires for taking shelter from the Ice Cold Darkness, because the cold stuff on Scalathrax would freeze you to death. Kharn took a flamer and torched everyone's shelters and started killing everything in sight. The World Eaters are now fractured into Warbands, who sell their services to other Chaos Armies for slaughter.
World Eaters use the most used catch phrase in 40k next to "WAAAAAAAAGH!!!!" or something for the Emperor. They scream BLOOD FOR THE BLOOD GOD in combat, while taking skulls for the skull throne.
In summation World Eaters:
Ruthless, bloody, and awesome. They are the Second most angry, and brutal Space Marines out there.