Don't kill Sinding.

What happens if you spare Sinding?

If spared, it seems Sinding is ultimately unable to control his beast blood. If the option to spare Sinding is taken, even if the Dragonborn turned around and killed him as well to complete the quest both ways, he can appear later in a random encounter as a hostile werewolf fighting a guard from the local hold.

Can you have Sinding as a follower?

This mod makes Sinding, the character from the daedric quest Ill Met By Moonlight, a follower.