In every generation, Assassins are real because theyre existed from Holy Crusade until today. It depends on "Assassination" of any political leaders. Some assassins in real world are bad, but some are good.
Assassins in their nature have existed for as long as there has been political opposition in any society. Assassins in the fashion of Assassin's Creed? I don't think so, those Assassins were formed in the 12th century during the Third Crusades and were snuffed out relatively early by Crusaders.
Yes but it is said that the creed is the only thing in the world that is idestructable as it only takes one person to start it up even if all its ormer memebrs are deceased, therefore if someone was to start the creed again it would be reborn.