janerowena I said that "I would never wish for anybody's assassination" but, as you pointed out, it did occur to me that Hitler, or someone of his ilk, may be an exception.
On thinking about it further though, as absent says, an act of this kind may well have repercussions that are even worse. The assassination of senior members of the Taliban and other dissident groups in Pakistan does not, in my opinion, make Pakistan a safer place but leads to revenge attacks and an ever increasing spiral of violence.
Also, you can kill one person but, as is commonly said, you can't kill an idea or belief and you may actually fan the flames to create something even more terrifying.
Stephen King wrote 11.22.63 in which he imagined someone being able to travel back in time to kill Lee Harvey Oswald before he killed Kennedy. From what I recall from a plot that became increasingly complicated, the result wasn't necessarily good.