I recently thought of a new book idea, science fiction no less – to wit:

Many people have often talked about going back in time to the 1920s to kill Hitler – this one comes to mind.  The time traveler always goes back in time before Hitler has committed any crimes.  People see the willingness to murder an (at the time) innocent person in cold blood as a sign of virtue, or resolve, or whatever.

Say this time machine exists.  Say also that time travel results in a reboot of history at the moment of travel.

Suppose WWII never happened, but time travel did – say you have someone totally wanting to commit cold blooded murder, but wants to do it in the past to escape capture.  Say that person goes back in time, commits many murders, but does not make it back to the time machine in time and gets caught.  Say that person goes on a killing spree of historical proportions, but of course no one gets that he is a time traveler from the future.  The state executes the serial killer Adolph Hitler.  History Reboots.

Say that in the future you have some other person who wants to kill someone, evade capture, and have it be somewhat moral.  Aha!  Let’s go back in time and kill a serial killer, but before they’ve done anything (when it will be easier).  This person goes back in time to kill Hitler, starts talking to him – discovers how much they have in common and the two commit many murders together as the best of friends.  History Reboots.

Say that happens again – and again (with many reboots).  With increased numbers, what started as a lone killer, turns into a friendship, turns into a gang, which turns into a political party, which turns into a mass movement as all of the future world’s psychopaths travel to 1920s Germany.  Their deep interest in murder morphs into Naziism (as something they can all agree to).  With each new member they get stronger, and draw ever more people to come nip them in the bud.

Not really sure how it ends, probably in something pacifistic, perhaps with a paean to salesmanship.