Someone who murdered a bunch of little kids. He's also purple and he is truly behind the slaughter. Beware, kids.
"Dude, did you hear about that song named The Man Behind the Slaughter?"
"Yeah... Uhm, I-" *in thoughts* : omg I must not reveal that I am the Ma Behind the Slaughter!
"BTW, your skin... It's, purple?"
"Oh, *to be continued*"
otherwise known as the purple guy, or as his canon name suggests- William Afton. The man behind the slaughter is a Fnaf meme where people point out purple people. Some actual backstory here: The man behind the slaughter, in FNAF lore, is a guy who killed five children to spark the start of Fnaf- hence his name.
Michael (purple guy's canon son)- My dad is the man behind the slaughter