Was going to agree with what another dude said below -- the fellow should be sporting the Raptor Imperialis (the eagle's head) and not the Aquila on his chest; if he's intended to be a Thunder Warrior.
Depends on hich source. Some say the Thunder Warriors got betrayed, others say they rebelled, and others had the Thunder Warriors asking to be given mercy kills after they had served their purpose.
There is a theory that Endryd Haar might have been a Thunder Warrior who had been adapted with Space Marine Gene seed perhaps as an experiment. Malcador called him an unusual case and an abomination. He had fought in the unification wars and was very different from other World Eaters.
Whaaaaaat, that's the first I've heard of Endryd Haar, I just give him a quick google and I am all over that. Unification stuff, like the missing primarchs, is wrapped in so much mystery (minus a certain Babu) that it's almost like a craving . Thanks for the heads up guys.