he might turn on sargeras, but he's not going to join us.
from what i inferred here, he felt a twinge of old feelings as he watched velen call him a monster. that's understandable, this is the guy he loved dearly for thousands of years prior. but then he buried it with rage, and when sargeras questioned his loyalty after all that he's done for him, he resolved to take the legion and achieve the vision he was given himself, without another failure plan by sargeras.
so he's still burning legion, he's just not exactly under sargeras' thumb anymore.