Yeah like Asana said the ‘betrayal’ was he acknowledged that what Sylvanas planned to do with Derek Proudmoore (someone who never did the modern Horde any wrong since he was killed by Dragonmaw red dragon riders before Thrall ever founded the modern Horde) was horrible (1. raising him into undeath against his will. 2. Planning on brain-washing him and using him as a mindless weapon to murder his own family) and helped him escape so he could be returned to his family.