Exiled for cruelty or somesuch sounds like a fitting reason for Bayel to leave Ascalon.
Edit: Or possibly insubordination. Possibly pressing an attack after being told to fall back, or slaughtering innocents or prisoners after being told not to. Any of those sound like Bayel's potential handiwork?
In Varesh's Kourna, he's found a position that lets him exercise his cruel streak, and get rewarded for it. I don't think he's necessarily in it for the good of Kourna, but rather for more selfish reasons.