when someone goes back in time to erase an event , wouldn't that mean they never went back in time. like they never used the time machine, so therefor they never changed anything, and then the event would happen. so that's why we cant time travel. or they are constantly creating alternate universe, that they end up going back to. but then there is the problem of them never had gone back, so maybe the person from the event timeline gets erased from existence. I don't know but this has been on my mind for a while.