Headcanon idk if it's actually canon :
-Passive Night died when he ate the apples but the present Nightmare we actually know kept not only some of his memories but also the traumas. Like, I can easily imagine that Nightmare would lose his shit if he saw people bullying a more younger or helpless person. He could even have trust issues if Passive actually got tricked in the past by villagers faking being kind. Those same traumas could also manifest themself as flashbacks.
- Nightmare would live and feel those flashbacks as his but they wouldn't actually be his traumas. It's just that by using passive Nightmare's skeleton as a vessel, he herited of the traumas. He sees those traumas and flashbacks as a weakness, a weight on his shoulder that he shouldn't even feel. They are not something a King of Negativity should feel.