AT the time the warmaster was chosen there were only 4 true contenders.
Angron & Curze were insane, Alpharius, Lorgar & Magnus was mistrusted by most of the primarchs, Perterabo was a petty bitter tyrant, Mortarian was weak willed(for a primarch) and a plodder, Fulgrim to much the peacock, the Khan to much the loner, Dorn to inflexible, Russ to much the barbarian killer, Vulkan felt to much & Ferrus to much involved with his machines.
That left Horus, Sanguinius, Guilliman & the Lion, Why did the Emperor choose Horus?
The Lion was a better strategist, the only Primarch to out do Horus as such & was privy to more of the Emperors secrets than any other primarch. Guilliman was a better organizer & would have dealt with the tedious details far better than Horus who hated that part of the warmasters job. Sanguinious was more charismatic, a better leader of men than Horus & was the one primarch more of the others would have been happy to see as warmaster than any other, including Horus.
So considering the consequences, why did the Emperor choose Horus as Warmaster?