Not so much the Honor of the Horde, as he had doubt about the wisdom and competence of Blackhand, and hated that Blackhand was controlled by Gul’Dan and the Shadow Council. His entire motivation for taking control of the Horde was to stop the waste that was Blackhand’s strategies, remove the influence of the Shadow Council for their betrayal of the Frostwolves (his friend Durotar), and conquer the World for the Horde.
It wasn’t until after the Wars in Azeroth that he learned of the Demon Influence that sparked everything, and by then he was too old to do anything about it. Until he came across a young Orc and began to teach him the old ways that he knew. Eventually giving him his Armor, and the Doomhammer, and the mission to rescue the Orc people.
Mannoroth had only revealed himself to Gul’Dan before we finally saw him in Warcraft 3 and learned that he was the one who gave the Orcs Demon Juice.