Terenas said it, not Tirion.Sargeras never was or will be a bad guy. He was the main hero of the WoW story, saving the universe from destruction.
It seems BFA is going to punt on the ending of the void lord story, but Azeroth may still be born a corrupted void titan and destroy the entire universe because our characters are evil selfish shits.
- - - Updated - - -
But some dipshit paladin whose plan failed until being saved by "the light" said something about more Lich Kings so he must be right!