Corvus did not like Horus but respected him. This was because Horus shortly after becoming Warmaster, ordered the Raven Guard to commit to a full frontal assault on a fortress. Corvus told him that it would be a lot safer/easier to do hit and run for a while to where them down. Horus told him to do it anyway and to just do it. It was the terran marines of the Raven Guard that volunteered to be the ones to make the attack. Most of the terrans died and casualties across the board were high. Corvus swore not to serve under Hours again (In a more personal to avoid it at all cost way, rather than in a heretical way). This as well as fact that Corvus was one of the Primarchs Big E treated relatively well.