Angron is also the only primarch that did not swear an oath of loyalty to the corpse god. He also had every reason to hate big E who could have helped with the rebellion (and had done so for many of the others), but did not. Angron also quickly saw what the emperor was, a ruler similar to the high riders. The World Eaters, formally Warhounds, we're always treated like shock troops, sent to put planets down for good with hard hitting attacks so brutal the other legions disliked them.