see the joke if funny because this i a reference to the hit game “Five nights at freddy’s” where a man depicted as purple, yes PURPLE HAH, is depicted to be the man who is behind all the killings that happen in the pizzeria you explore in the five nights at freddys games, the reason he is important is because the killer animatronics roaming around the building are possessed with the souls of the children that died and he himself also at some point dies and becomes one with his creation. now the reason you will see him alongside the phrase “Man behind the slaughter” is because of the MLP song maker The Living Tombstone who created several fnaf songs including one know as the “fnaf 2 song” a song based on the second game in the five nights at freddys series in which a mom of one of the killed children mourns their death via song in which one of her lines is “It’s been so long since last i’ve seen my son lost to this monster, to *the man behind the slaughter* referencing the purple man from the five nights at freddys games. finally because of his correlation with the color purple many purple objects can be give the “man behind the slaughter” phrase and it can be considered quite humorous! and there you go I have explained the joke! have fun!