Should an Unseen Assailant atack a model and score a kill with the first action, he or she may elect to use their remaining actions to immediately move to a position of safety. If that position breaks LOS with all the enemy models within 12" of the unseen assailant, their stalking status will be preserved.
What about if he scores a kill with the second action, e.g. after using the first one to move: can he use the third action to fulfill the LOS breaking requirement and return to stalking status?