We assume the kill-Hitler scenario would work because that era is so well-documented and we have many first-hand accounts of what happened (let's face it, The History Channel owes its success to World War II's thorough records), so a prospective time-traveling assassin would be able to pinpoint the perfect moment to swoop in and complete his mission. But even though those events took place less than 100 years ago, there are still many gaps and unsolved mysteries of the era.
So...what if someone went back in time several centuries, even thousands of years, with a similar goal? What if the man they try to kill had been prophesied? Can one really kill a prophecy to save history?
Ironically, we rely on history to help shape our worldviews and often our own identities. But can we really know everything in history? No. It's impossible. Just think of all the mistakes one over-confident person could make if they were actually able to go back in time.
Suddenly that whole kill-Hitler thing doesn't sound so simple. Nether is killing The Evenarian.