I tend to spare him because, despite the fact that he's dangerous I consider him a victim of circumstances. I never found him again after Ill by Moonlight in previous playthroughs, but according to his page you can find him around corpses in a bandit camp or near Helgen.
I think you can justify both killing him or letting him go. Thoughts?
I spared him too as he is very remorseful for the murder he committed, which he had little control over. And the only reason he took the ring was to help him control his transformations. So it's possible he might be hoping for a cure as well.