Narcissists are cruel from day one, the only problem is most do not recognize this. A person can be cruel without appearing cruel. Lack of accountability, manipulation, love bombing and lying are all examples of “unseen” cruelty. So to phrase such a question in a different way- “why do narcissists tend to appear more cruel after a breakup?”.
Narcissists punish their victims in very cruel “unseen” and “seen” ways and for various reasons. It gets to the point in a “relationship” with a narcissist where the narc removes their ugly masks one by one, for you to see all. The narcissist does not care about you seeing how directly cruel he is being because 1. It will be your word against his at the end of the day 2. He creates justifications to why you deserve to be abused (example-he is frustrated with you because your so mentally ill), 3. Narcs like to divide and conquer 4. Your trained to keep your mouth shut for the most part in fear of looking crazy 5. Your misery is worth a lot (not his facade though because he believes it is so powerful, nothing can crush it) 6. He increases the mind control by intermittent reinforcement 7. In his mind, there is no breakup-there is master and slave, that is all