Sam the Butcher. All part of his twisted long-term plan to get his hands on Carol. He even seduced Alice the housekeeper so he'd be in better position to "take care" of Mike. And what became of Mike, you ask? Well, the butcher shop did have that special on ground beef that same week...
And if that episode had ever aired, it would be my favorite TV show ever! (Do not get the love for the Brady Bunch at all.)
Interestingly enough, the original draft of '90s movie had Jan gunning down the rest of the family after one too many times of them paying attention to Marsha.
The Sam theory is compelling, but the problem is that he didn't start the ground beef special until after the orange hair incident. What is most suspicious, is Mike's complete disappearance after the penultimate episode. Clearly that medling kid Oliver did it, since he wasn't getting as much attention from Mike as the real Brady kids.
He had naturally curly hair and when they went to Hawaii, it started curling because of all the humidity. The female cast members loved it, and insisted that he stop straightening his hair.