Skyrim Ill Met By Moonlight Sinding attack me after killing hunters
Solution 1:
There are multiple things to try.
Exit the area. It is possible that the aspect of Hircine will show up, and you can automatically advance to the next part of the quest.
Exit the area, wait for 24 hours, reenter and talk to Sinding. Most NPCs calm down after a while.
Cast a calm spell. The illusion school has many calm spells. You can buy them from Drevis in the College of Winterhold. If you are an imperial, you can use Voice of the Emperor. It is also possible that the Kyne's peace shout will calm Sinding.