It seems that the reason Arha left the Striking Scorpions is because he didn't like being the boss of a temple, he just really liked (and was really good at) killing stuff. It would thus make perfect sense for him to join the Incubi but refuse a leadership position.
That being said, this argument made a lot more sense in the old codex. There the Incubi had clear ties to the Striking Scorpions (the helmets being the most obvious) that seem to have largely disappeared from the current codex.
"Look into my eyes, and see your death."
"Let them hate, so long as they fear."