"In chaos theory, the butterfly effect is the sensitive dependence on initial conditions in which a small change in one state of a deterministic nonlinear system can result in large differences in a later state.
Others have commented the assassin is a time traveller. Time travel has nothing to do with it! Affect an unrelated thing in such a way that the knock on effect leads to the death of the intended victim.
The Butterfly Effect. Little things in the past can make big differences in everyday life. If you time traveled to the past and stepped on a butterfly a thousand or a million years ago, who knows how history would be changed?