
You have an aversion to death, and the idea of taking a life chills you to the bone. All damage that you do is considered non-lethal. Unconscious creatures gain the effects of the Sanctuary spell against you while they remain at 0 HP. If you ever do take a life, you must make a Wisdom saving throw, or be stunned until the end of your next turn.
FLAW. This is a flaw feat. At your DM's discretion, you may take this feat at level 1. If you do, you may also gain a feat, or increase two of ability scores by +1 (or one by +2).

Of what DC?
I personally recommend maybe making it DC 10, or half of the damage of the killing blow, whichever is higher, like concentration.
I didn't want to set a hard DC, I thought that should be for the DM to decide.
I would suggest dc 12 with disadvantage, otherwise you will get people with a max wis reap the benefits from this without taking any real disadvantage.
If that's your thoughts, then perhaps you should read what [spell[Sanctuary[/spell] does. You literally can't attack that target while it's at 0 hp. Besides, setting a hard DC is difficult to balance, which is why it'd be more appropriate to do something like "DC 8 + # of hit points putting them past fatal". Then if you do a small amount, your character can "shake it off" and "pretend it didn't happen". But if it's massive damage, like say 10 or more, then your character would definitely have a hard time dealing with it.