 The problem may be better understood in following way: each time you try to retro-kill somebody who is not alive after the time the purported effect of killing took place, your assassination may still fail because of your hand was shaking, etc. Such particular facts explain why you actually missed the target which you in principle were able to hit. But to say that you are in principle able to perform retro-killing means that there are laws of nature that normally allow you to perform such an action in the appropriate circumstances. Similarly, each time you try to retro-kill somebody who is alive after the purported death took place, you may fail for one reason or another. But you must always fail to retro-kill somebody who is alive after you did your action, i.e., you are in principle unable to retro-kill such a person. In those cases it is physically impossible for you to kill, say, your younger self. It seems, therefore, that there should be some laws of nature, working on either a local level or a global level, which violate such an action and makes it physically impossible.