There's another theory about the foretellers being the opposite to the sins they got their names from, which makes more sense in my opinion.
Ava is kind and selfless, Aced is prone to take action, Ira is calm and thorough, Gula is cold and reflexive, Invi is fine with just observing instead of desiring a more important role and Luxu watches from a distance instead of acting impulsively.
Noone truly knows what the Master of Masters is actually thinking, but talking to Luxu he admitted manipulating the foretellers so they had doubts and negative feelings in order to make them suitable as vessels for the 7 darknessess, but also trained them to have strong hearts so they don't lose themselves to darkness.
In case he was being sincere, I wouldn't say he's prideful. I think he feels some remorse for tricking his disciples but did it for the greater good.
Who knows, maybe the names of the sins fit the personalities of the 7 darknesses once they manifest.