NPR lays out the scenario:
You find a time machine and travel to 1920. A young Austrian artist and war veteran named Adolf Hitler is staying in the hotel room next to yours. The doors aren't locked, so you could easily stroll next door and smother him. World War II would never happen.
But Hitler hasn't done anything wrong yet. Is it acceptable to kill him to prevent World War II?
Leaving aside the highly dubious notion that World War II could be prevented with such a simple act or that the results would be more favorable than World War II as it unfolded in real history, you have a moral dilemma.
Rebecca Friesdorf, a graduate student at Wilfrid Laurier University in Ontario examined how people respond to this and other moral dilemmas and their reasoning for their choices. She surveyed 6,100 people from the USA, Canada, and Germany. Friesdorf published the results of her study in the Personality and Social Psychology Bulletin.
Friesdorf found that women were less likely to kill Hitler when given the chance. Ben Tufft writes in The Independent:
While men and women both calculated the consequences of their decision and computed how many lives might be saved, females found it harder to commit murder and were more likely to let Hitler live.
-via David Burge
"Would it be acceptable to preemtively kill a person you for some reasons are sure will perform terrible atrocities in the future."
I hope everyone will be reluctant to kill a newborn for a crime it will commit in his distant future.
On the other hand i expect that nearly everyone will command a sniper targeting a suicide-bomber to pull the trigger without much hesitation.
For me, it just depends on time periods and causality.