You can stop wringing your hands and wetting the bed. Studies show that justified killing leaves few scars, if any, whether done to protect one's self or others. But I find it interesting that you believe you'd have nightmares if you defended yourself, but not if you defended others. It's that kind of self-contempt that usually leads people into collectivist groups - the same kinds of groups which, historically, have led to the murder of more hundreds of millions of their own people than all the wars combined.
And THAT gives ME nightmares.
Sorry... but you must have never been in the Service. Killing anyone always causes nightmares and problems... Good for you in realizing that you would not have such problems..