Killer was always a trouble maker, he always did shit which pissed Nightmare off but lately.. Nightmare hasn't been acting like himself anymore. He started to be way too merciful than a 'king of negativity' should be.. it's annoying Killer. Nightmare hasn't done anything even when Killer deserved punishment. So Killer decided to be more annoying..

"HEY BOSS. I ACCIDENTALLY THREW THAT STATUE OVER YOU LIKE SO MUCH AND IT BROKE TO PIECES."