Well, first of all, as a primer to this sort of thing, I highly recommend “The Theory And Practice Of Time Travel,” by Larry Niven.
Y’see, there are a variety of factors to consider, here. First of all, is it possible to alter the past?
The past has HAPPENED. It is IMMUTABLE. Once it’s happened, and led to a future, you CANNOT change it. Some time travel stories are based around this. In one that Niven cites, a man travels backwards through time to prevent his own assassination. He succeeds… only to have a meteorite the exact size and shape of a bullet smash into his head.
Time will repair itself, no matter WHAT we do to it.
ON THE OTHER HAND: what if that’s not true? What if it IS possible to change the past, assuming you’re there to be a part of it? This brings up the “Grandfather Paradox” – you travel back through time and kill your grandpappy before he met your grandmammy, thus preventing the birth of your father, thus preventing YOUR existence… which means you couldn’t have traveled back through time to kill grandpappy, which means…
You get the idea. In the “Immutable Universe” theory, someone else, someone incredibly similar, will take Grandpa’s place, and history will continue unimpeded. In fact, it might be YOU – you are, after all, right there, and you’re already carrying the old man’s genes…
The only theory I’ve ever seen that I really liked here was this: Time travel IS possible… but if you travel back through time and shoot your grandpappy… and then come BACK… you will find that nothing has changed. In fact, if your grandpappy was alive when you LEFT, he will still be alive when you come BACK, despite the fact that you SHOT the bastard, forty years ago!
How is this possible? Simple. When you went back and shot grandpappy, you simply caused the generation of a parallel universe. In one timeline, the original one, your grandpappy is still hale and hearty. The timeline YOU created, the one in which he is dead, is now a “Branch Timeline” and does NOT lead to the future in which you were born.
…which means that unless your time machine has a firm anchor to the reality that you left to shoot grandpappy, life could be very interesting when you get back. What if you get back to the ALTERNATE future, instead of the real one? You are now in an alternate reality in which you never existed, and have no identity… an interesting possibility, assuming you weren’t real anxious to go back and resume the life you left behind to begin with.
Of course, that means that if TVGuy went back and dumped a bottle of bleach in the primordial soup, he’d simply be creating a parallel reality in which life didn’t evolve on earth. Of course, when he tried to get back to his future, he’d simply be going to an equally sterile future. Eventually, he’d die, and then the bacteria in his body would go to work on his corpse, spreading to the outside environment, and finally, spreading into the primordial soup, and then…
TVGuy would be, in some twisted sense… God.
Man, my head hurts. I’m going to go and get a big drink, now.