Fred Phelps's son has written a very disturbing article about life in the Phelps home: there was a lot of abuse. I can't imagine that being constantly filled with rage and hate is somehow going to bypass your family, even if you would hate anyone else who hurt them. Of course, that's not Christianity.
But within the ranks of those who call themselves fundamentalists, which doesn't exclude all the Reformed churches, and whom we would be inclined to call brethren in the judgment of charity, there are certain dynamics that promote abusive situations. For one thing, there is often a great deal of value placed on reputation: whether it be as a "testimony" to the ungodly or as an "example" to other believers; that in turn can create tremendous pressure to keep up appearances. That means that the perception of your godliness (which many people can't distinguish from the thing itself) is tied up with the behaviour of your children - so you keep them in line at any cost. Add in to that the vivid conception of their danger of hell from sin, and that disobedience is sin, and very radical measures no longer seem inappropriate.
The keeping up appearances dynamic also plays into situations of church discipline.
I'm sure there's more that goes into it, but you can see from that how even sincere people could easily go too far; the answer, of course, is a practical reliance on the grace of God and submission to His will, with a repudiation of the fear of man.
