Brace yourself for this question. It is a really complicated and confusing question with an even more confusing answer.
If a man goes back to the past and kills his grandfather, what would happen?
Look at the right of this page.
The answer to the question is if he goes back and kills his grandfather, he doesn't exist to kill his grandfather which means he couldn't have killed his grandfather which means he would be alive to kill his grandfather. Basically, it goes on forever.