If you go back in time to kill your grandfather, you would pop out of existence, but if you pop out of existence your grandfather wouldn't die. but then you would pop back into existence to kill him again. so would it be an endless loop? me figure this out

I bet that is actually true. But where in the world did you get this idea of?

No. That isn't specifically how time travel works.

When you time travel, you are taking your body from your current time and traveling through universal zones to go back to the specific time you wanted. You would just be absent from the present time as if you were at home, or something.
You would go back into time and you would have to be able to keep yourself hidden enough, because if anyone recognized you (such as family), it would alter time itself.
Along with, if you killed your grandfather then your parent (from such grandfather), may or may not exist, which could alter your own existence, changing time and space itself, altering the universe itself.

So no, it would not be an infinite loop. You would just be absent from your present time until you decided to return back to it.