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So on that logic if you buy a gun and judge It must be used to kill a random person on the street thats ok? Supposing there were no law against that

Depends on the situation. If that person was baby hitler and we used false analogies, then the answer is quantum blue.

And how would you know exactly that was baby hitler without traveling in time?

It's best not to think about it, really. You end up being your own great grandfather.