Chronicles 3 did shed some new light on Bolvar and the LK himself.
He's been gradually losing his grip on the Scourge and Ner'zhul is still in there somewhere (and he still feels remorse for his role in damning the Orcs, so he's still pro Orc). I can actually see him losing total control and the new 'wrath 2.0' expac or patch will have Bolvar actually help us kill the Scourge once and for all.