Clearly, the present never is changed by mischievous time-travellers: people don't suddenly fade into the ether because a rerun of events has prevented their births - that much is obvious.
Hmmm... I still don't understand; If someone went back and killed Marx's grandparents, Marx's contemporaries wouldn't suddenly see him "fade into the ether." He just simply would never have existed for them. And we here in the 21st century wouldn't be aware that anything amiss had happened either. In fact, couldn't time travelers be traveling backward in time constantly killing people whose offspring they don't like -- thereby instantly wiping out our memories of those people and anything they might have accomplished during their lifetimes? I just wish they'd get around to Stalin, Mao and Hitler!
Just to be fair, they did kill Zarquon, Lifnerd, and Bullawayo -- and they were far worse.
Ah, but you missed the point! Some time travelers already have taken care of evil tyrants who would have been worse that those three, but the Universe will not be denied, and Stalin, Mao, Pol Pot, and Hitler rose to take the place of those who were removed from history. Didn't you see the Outer Limits episode where a time traveler went back to kill Hitler as an infant, but it turned out someone switched babies in the end?