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

Is it better to spare Sinding or kill him?

Don't kill Sinding. Instead, help him kill the hunters. ... Now exit and Hircine will appear again in his stag glory to ask you if you killed Sinding. It doesn't matter if you pick "I failed" or "I disobeyed your orders" he will still commemorate you about turning the hunt around (hunters become the hunted).

What happens if you let Sinding live?

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.