Alectai
Speaks Words of Wisdom... On occasion
I don't know, being some kind of Martial Paragon on top of being a Master Psyker could push Serras into the "too good to be true" area and deemed an acceptable loss to destroy.
He didn't kill Magnus for literally having bargained with the powers of Chaos and given up his eye for it.
The Emperor does not consider Chaos to be a threat at this time.