[Sethomas]

If someone went back in time to alter the past then we'd know about it from the past. Hence, if you wanted to destroy the universe with the grandfather paradox, you'd be wise to ask your grandfather if he remembers any strange encounters with someone resembling yourself. If he says "no", then it's obvious that you will never undertake a mission to the past that involves him, so you fail. If he says "yeah, I kicked his fucking ass and then he died", then you will have the interesting experience of living out the rest of your days with the known and immutable fate of getting killed by your grandfather with all imagined conceptions of free will being shattered.