This is the nickname that is given to the "Metal" music genre by those who appreciate real music instead of this false genre which can barely be referred to as "Music". It is given this name because the music itself is just as horrible as killing puppies.
People will often try to defend their so called "Music" in every possible way because they are too afraid to realise that it is not music, as their whole world of hating and suicidal behavior will have been wasted.
Emo: *Starts playing metal*
Normal Person 1: Dude what the hell is that noise?! My ears are bleeding!
Normal Person 2: Killing-Puppies-Music.