"Impulsivity is the devil" is a way to say in Engpsh that acting on impulse can lead to negative consequences. Impulsivity refers to the tendency to act or speak without thinking or considering the potential consequences, and it can sometimes lead to problems or regrets. It is often compared to the concept of "the devil," or a destructive or tempting force, because it can cause people to make poor decisions or act in ways that they later regret. It is important to try to think before acting and to consider the potential consequences of one's actions in order to avoid negative outcomes.