HG Wells' Time Machine - at least the updated stupid version - looked at the impossibility of changing the past. Certain events have to take place in order for you to actually time travel in the first place. Does changing them mean you never could have gone back in the first place?
Unless time travel had a delay, in that the consequences of changing the past didn't take place immediately, it would be logically impossible to go back and kill your own parents. You'd have to trust that the past would somehow prevent you from actually doing it.
Still, if Blackadder taught us anything, it's that time travel exists solely for the purpose of going back far enough to punch the crap out of Shakespeare. I'm sure that's something we've all wanted to do at some point.
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