Changing Time, Doctor Who Theory
In Genesis of the Daleks, with Tom Baker as the Doctor, he had an opportunity to erase the villainous daleks from existence, but hesitated because the threat of the daleks had united warring factions into a common cause.
Also, when David Tennant as the Doctor went to Pompeii, he learned he and he alone would have to destroy Pompeii by releasing the build-up the volcano, otherwise the entire world would be destroyed. HE would be the cause of one of the single most cataclysmic events known to man.
It's interesting to think if we could go back in time to kill Hitler or stop the sinking of the Titanic, we really don't know what it is that may be unleashed.