I'm not really seeing insults. He is insulting how Thrall is running the Horde, suggesting how he would do it better, but doesn't direct anything at Thrall specifically until Thrall attempts to step down from the Challenge (which any Orc should not be doing, you do not back down from a challenge). Which is "Hey what the hell you are talking about ! you are so wrong in my personal opinion leading should be bla bla bla = opinion + suggesting" which yourself have implied is acceptable.
Also, i'm asking for things before Mists, so that you can show how he was always bad and there were clear signs. Pointing to current and then going "see he's bad now so he was always bad" isn't proof.
As to Magatha, perhaps he felt that she was Baine's kill to have, perhaps he was in a pissy mood, perhaps he wanted to deny her the honor of death in combat. In what way does him not chasing down Magatha with the fury of a thousand suns prove that he was always bad and was a clear sign that he would become bad?