Dreaming of an assassin is usually a warning, indicating that your opponent will launch an attack on you or launch a conspiracy, reminding you to be vigilant and prepare more to avoid getting into trouble and suffering losses.
If you dream of an assassin trying to assassinate you, it implies that what you are doing will suddenly encounter unexpected resistance and may fail.
Dreaming that someone else was killed by an assassin indicates that you may encounter disaster or be implicated in unfortunate things.