Children can be bad when the parent is a loopy themselves. This is one of those stories with such possibilities.
Frances was a normal kid, maybe. With three murders under his belt and his older brother in the looney house after setting neighborhood dogs on fire. Frances is not even a teenager yet and is also missing the biggest part of his life that his father is holding with his secret.
This was a blowout ending, I really thought myself that his older brother was his actual alter-ego, but I guess I was wrong.