If Horus was killed before the Siege of Terra...

[BJORNin83]

Since they are following a certain god, Fulgrim, Angron, Magnus and Mortarion wouldn't be the warmaster.

does mortarion already follow a single god? if i recall correctly he only went over to nurgle after typhus released the plague on their way to terra. if he coud unite all the legions behind him i can see all the gods possibly investing in him

yup your right. They didn't get infested with the plague until Typhus killed their navigator and the ship ended up in the warp and unable to get out on their way to Terra.

[Sgt.Sangha]

everyone keeps pointing out how they all lack leadership ability, i doubt that. the emperor designed them to be leaders and generals, and for the most part they are great leaders during the crusade. Fulgrim and his legion have been known to have some of the best discipline and purity prior to their fall. In fact it is due to this discipline that the majority of them went along with their leaders in the first place. Chaos took advantage of the superiority complexes of the high command and used it to gain the whole legion. I believe Tarvitz was the ideal Emperors Children marine, who did exactly what was expected of him for no other reason then the simple fact that it was necessary to do so. I remember reading this in a white dwarf article, I believe it is also in their IA.

Although if you read fulgrim, in the battle with the lear they suffered heavy casualties and had to spread propaganda around the fleet to make it look like they had very little resistance, and this was all due to Fulgrims grandisment. but these could be noted as small cracks in their character that developed due to where they grew up. However that still does not mean they were not disciplined astartes with high skill levels, and with a Primarch who could lead them well.

the same can be said for Mortarion, again a very strong legion that until the warp incident was a well diciplined legion, remember they did not run for the eye of terror, but instead did efficient tactical retreats

[Son of Rawl]

Primarchs were made to be the greatest generals their was no doubt about that, the problem though is they all have extremely huge egos. Horus was grudgingly excepted as warmaster as everyone loved Horus, on the traitor side of things their is very few that people that will blindly follow from all legions, Lorgar is indeed the chosen of chaos but at this time chaos worshipping was not clearly in the open, this may of changed though.