In order for the people alive today to be here, history, down to the last dotted i and crossed t, had to have happened exactly the way that it did. If during copulation, your father had ejaculated half a second sooner or later than he did, you would not exist. If your mother had been in just a slightly different position, you wouldn't exist either.

It's not good enough to say that a person is the product of the sperm and eggs of their parents, but they are the product of one specific sperm and one specific egg. Had any other sperm out of the several hundred million released by the father made it to the egg first, someone else would have been born. Instead of going back in time to kill your own grandfather, thus preventing your birth, all you would have to do would be to stop him on the street on his way home on the day of your parent's conception and ask him for the time. This tiny delay would set into motion your own undoing.

So while I do sympathize with the suffering of past peoples, I'm still glad that it happened. Otherwise I would not be here.