Attempting to alter large, significant historical occurances with one bullet would not work, because you're only destroying the opportunist, not the catalyst; for example, with no Hitler, someone else would've seen the opportunity to lead Germany at its most vulnerable. Kill Edison, and eventually, someone else with the capacity for invention would recognize the need for better communication. History usually doesn't happen because of people, but because of events and circumstances. By killing someone like Hitler, the best you could hope for is someone less diabolical to fill the void that his death would create.
Smaller-scale assasinations, on the other hand, would work better; killing someone like Mark David Chapman would've almost certainly enabled Lennon to live for at least several more years barring a freak circumstance (like Chapman was in the first place). It depends on the context, though; someone like Notorious B.I.G. would probably have died from a different assasin even if you killed his first murderer simply because of the feud going on at the time.
