>Once you bring in "possible", you have to consider whether a copy of "you" could spontaneously coalesce in front of your grandfather with a gun in your hand and shoot him, and then maybe dissipate right after, or not.

According to Everett of quantum mechanics, it is another you killing another of your grandfather so you won't dissappear from the current world.

According to the Copenhagen interpretation, there's a high chance you dissappear.