Redheaded Stepchild

I am wondering where the phrase "I'm going to beat you like a redheaded stepchild" came from. Where did the whole idea come from that a redheaded stepchild is a bad thing? Thanks for any help anyone can give.

From a previous discussion ("stepchild: under Archives):

A stepchild might be singled out for abuse. But a red-headed stepchild (who presumably looks like his or her absent birth-parent) might be abused even more because he or she is so obviously different from the other children.

The expression I have heard is, "I'm going to beat you like a red-headed stepchild."

Sad, isn't it?

Only slightly more humane is: "beat you like a rented mule."