I didn't know you could let him live.
yeah, you can choose to be persuaded that he wasn't in control of his actions and let him live. He'll hang out in the Retching Netch and you can have him as a follower, he killed Teldryn Sero in my game after a strange turn of events I still don't understand though, so if you like the sellsword you might not want to keep Ralis alive.
Teldryn Sero? Is he the guy you can hire?
yes, yes he is.
I used him as a sacrifice for the Daedric quest to get the Ebony Mail.