The Grandfather paradox.
Notorious and revered mallorys_camera raised an interesting point. Would you go back and assassinate Adolf Hitler? The thing is, by changing time it could bring about something worse. Personally I'd opt for Oliver Cromwell. He did the Shoah, but for Irish folks. I am trying to recall a Czech Sci Fi film "If it is next Tuesday I'll be killing myself" or some such. My Maploco doesn't seem to work, naughty Harcourt Street.