Random head cannon:
I’ve hinted at this in my writing, but when Nightmare was little and being bullied, he went into freeze, then flight, then fawn reaction. He got beaten up and was scared to do anything against it.
But now that he’s older and corrupted, when something happens that reminds him or makes him feel like when he was younger and being bullied, he doesn’t get small or go into the freeze reaction and gets all up in his head and feels like he’s six again.
Instead, he gets angry and fights back. He is not that weak little kid anymore and he said that he wouldn't let anyone treat him like that again. He is a guardian, a king, a god. He refuses to be like when he was younger, he refuses to be walked all over. So much so, to the point it becomes irrational. He has yet to face any consequences yet because he just gets so angry he doesn’t keep his strength in check and ruthlessly kills whoever was teasing him to begin with.